Nearly every “best cold email infrastructure” list is a ranking. Ten vendors, a star rating, a winner. That format is useless here, because the vendors on those lists are not competing products.
They are three stages of the same journey. Almost every outbound team starts in stage one, a lot of them detour through stage two, and the ones still sending at volume in two years are in stage three. Picking the stage you are actually in matters far more than picking the vendor inside it.
The disclosure, up front: we build Mission Inbox, which sits in stage three. We are not neutral. What we can do is source every competitor claim to that vendor’s own pricing page, help docs, or a dated public review, and be specific about who each stage is genuinely right for, including the readers who should not be buying from us yet.
The three stages, at a glance
| Stage 1: Google resellers | Stage 2: Azure tenants | Stage 3: Dedicated SMTP |
|---|
| Who is here | Almost everyone, at the start | Teams chasing cheap volume | Teams who outgrew or got burned by 1 and 2 |
| Examples | InboxKit, Zapmail, Cheap Inboxes, Premium Inboxes, ScaledMail | ScaledMail, HyperTide, Inframail, Sending.ac | Maildoso, Mailforge, Infraforge, Mailreef, Mission Inbox |
| Typical price | $2.50 to $3.50 per mailbox/mo | $25 to $50 per domain/mo | $2.40 to $4 per mailbox/mo, or a platform fee |
| Who owns the rails | Google | Microsoft | The vendor |
| Why you leave | Volume ceiling, suspension waves, no diagnostics | Tenant blocks, disconnections, TERRL caps | You don’t, if the vendor warms its own IPs |
| What ends it | An upstream policy change you had no say in | A tenant block that takes 25 mailboxes at once | Nothing external. Your own sending behaviour |
The thing nobody tells you before you buy
Here is the sentence that explains stages one and two.
Every vendor selling you Google mailboxes or Microsoft mailboxes is a reseller of somebody else’s infrastructure. Not most of them. All of them. It is not an insult, it is arithmetic: only Google provisions Google Workspace, and only Microsoft provisions Microsoft 365.
Premium Inboxes says so in their own FAQ, and credit to them for it: “Premium Inboxes are a reseller of Official Google workspace and Microsoft 365 Business inboxes, built specifically for cold email marketing.”
What follows is the part that costs people money. If your mailboxes live inside somebody else’s upstream account, a policy change or an enforcement wave at that upstream provider takes your sending down. Not degrades it. Takes it down, overnight, with no appeal, because you are not the account holder. You are a line item inside one.
Why rented rails work at all, and why that same reason is a ceiling
People assume Google and Microsoft mailboxes land better because Google and Microsoft are prestigious. That is not it, and understanding the real reason tells you exactly when they stop working.
Google is a shared IP environment. So is Microsoft. Neither hands you an IP unique to your account. By the logic most infrastructure marketing uses, that should be a disadvantage.
It works because of dilution. An enormous volume of ordinary business email flows through those servers every day: people replying to customers, sending invoices, booking meetings. Cold outbound is a rounding error beside it. Your prospecting is hiding inside an enormous crowd of legitimate mail, and that crowd is carrying your reputation for you.
Read that again, because the implication is the whole article: on rented rails, the reputation protecting you was never yours. You are borrowing it. Which is fine, right up until the moment it is withdrawn, and you have no ability to influence, inspect or rebuild it. More on the mechanics in our breakdown of dedicated IP vs shared IP.
Stage 1: Google Workspace resellers
The vendors: InboxKit, Zapmail, Cheap Inboxes, Premium Inboxes, ScaledMail, plus Icemail, Primeforge, Mailzy and a long tail. They sell the same thing: Google Workspace mailboxes provisioned in bulk with DNS automated for you.
| Vendor | Price/mailbox | Setup time | Support | Trustpilot |
|---|
| InboxKit | $2.50 to $3.50/mo | “Under 10 minutes” | Email and chat | No profile |
| Zapmail | $3.00 to $3.50/mo | “5 minutes average” | Chat and phone | 4.5 (87 reviews), 7% 1-star |
| Cheap Inboxes | $2.80 to $3.50/mo | “Same day” | WhatsApp, 24/7 | 4.5 (30 reviews), 7% 1-star |
| Premium Inboxes | $2.80 to $3.50/mo | “Under 6 hours” | Slack + account manager | 5.0 (494 reviews) |
| ScaledMail | $3.50/mo | 24 to 72 hours | Dedicated Slack channel | 4.7 (38 reviews) |
What actually separates them
Very little, and that is worth knowing before you agonise over the choice. Feature lists are interchangeable: automated SPF, DKIM and DMARC are the baseline, sequencer integrations, admin panel access, bring your own domain. Every vendor claims all of it and most deliver it.
Setup time is a genuine spread. Zapmail advertises five minutes, Premium Inboxes under six hours, ScaledMail 24 to 72. That is a 30x range on a number all three put in their hero, which tells you buyers shop on it.
Support dominates the reviews. Cheap Inboxes runs support over WhatsApp with sub-five-minute responses. ScaledMail gives every customer a dedicated Slack channel. Read the five-star reviews across this bucket and they are almost all about a named support rep, not the product.
The third differentiator is the one buyers do not price in: account and DNS stability, and who controls the tenant. One Zapmail customer in Germany, July 2026: “They regularly disconnect the domains from our webhost where our website lives and then we are sending mails with broken links because the website is down. This happens every few months.”
And the clearest tell in the category: Premium Inboxes sells a $7,500 per month tier whose headline value is essentially reconnection babysitting, with an SLA promising soft-disconnects auto-reconnect in minutes. If mailbox stability were a solved, commoditised feature, that tier would have nothing to sell.
Where the cheap seats come from
There is a way to source Google Workspace seats below Google’s $8.40 list price, and it involves licences never meant for commercial outbound: education seats, nonprofit seats, and legacy free G Suite accounts, sold through what the industry calls “panels.”
Google began issuing permanent suspensions against these in October 2025, and enforcement was still sending notices in March 2026 with hard compliance deadlines. If you are buying Google mailboxes from anyone, this is the single most important thing to verify before you pay, and it takes about thirty seconds.
We wrote the full checker as its own guide: how to check whether your mailboxes are panel accounts. Background on the enforcement wave itself is in our write-up of the Google EDU panel ban.
The ceiling on stage 1
Google works. Then it stops, usually for one of four reasons: per-seat sending limits you cannot raise, suspension waves you cannot appeal, engagement-based filtering you cannot see into (see Gmail’s Gemini filtering changes), and the fact that you cannot diagnose a placement problem because the reputation is not yours to inspect. We covered the migration decision in depth in why teams are moving cold email off Google Workspace, and the head-to-head in Mission Inbox vs Google Workspace.
Stage 2: Microsoft mailboxes and the Azure tenant trick
Microsoft splits into two very different products sold under one label. Real Microsoft 365 accounts are licensed seats, a handful of mailboxes per domain. Azure tenant setups are a vendor spinning up an Azure or Entra tenant, attaching a domain, and creating 25 to 100 mailboxes on that single domain. Most “cheap Microsoft inboxes” are the second thing.
| ScaledMail (Microsoft) | HyperTide | Real Microsoft 365 |
|---|
| Price | $50/domain/mo | $25/domain/mo | ~$8.40/seat/mo list |
| Mailboxes per domain | 25 | 25 to 50 | Normally a handful |
| Model | Azure tenant | Azure tenant | Licensed seats in your own tenant |
| Admin console in your name | No | No | Yes |
| Trustpilot | 4.7 (38) | 3.2 (1 review, unclaimed) | n/a |
Microsoft’s own position
Before any competitor criticism, read Microsoft’s documentation: “aside from the High-volume Email offering, Exchange Online doesn’t support bulk or high-volume email.”
That is Microsoft stating the whole Microsoft-mailbox cold email category sits outside supported use. Everything built on top is built on a tolerance, not a product.
TERRL, and why stacking mailboxes buys you less than you think
Through March and April 2026 Microsoft rolled out the Tenant External Recipient Rate Limit, capping total external recipients per tenant per day. It scales sub-linearly:
| Licences in the tenant | External recipients/day (TERRL) | Per-licence equivalent |
|---|
| 1 | 10,000 | 10,000 |
| 10 | 12,006 | 1,201 |
| 25 | 14,259 | 570 |
| 100 | 22,059 | 221 |
| 1,000 | 72,446 | 72 |
Read the right-hand column. One licence gets 10,000 a day. A hundred licences get 221 each. Stacking mailboxes into a tenant does not multiply capacity, it divides it. The entire economic premise of this model is in tension with how Microsoft meters sending.
- New tenants are throttled on arrival. Microsoft reduces the quota during onboarding. Freshly provisioned tenants, which is the whole product, start restricted.
- onmicrosoft.com domains are capped at 100 external recipients per 24 hours, NDR 550 5.7.236.
- A breach blocks the entire tenant, not one mailbox. NDR 550 5.7.233. One careless mailbox takes the other 24 on that domain with it.
The vendors admit the stability problem themselves
We do not have to make this argument. ScaledMail makes it on their own site about a competitor: “50 inboxes per domain increases Microsoft tenant block risk... at higher density, you’re more likely to trigger ‘unusual activity’ account and tenant blocks from Microsoft.” And they concede they are on the same architecture: “both use Azure tenants, but we’ve found that 25 is the stability sweet spot.”
The most revealing evidence we found is a five-star review of ScaledMail from a long-time customer: “Always responsive and handle disconnected inboxes quickly.” A happy customer at the best-rated vendor in the category describing routine inbox disconnection as normal service. Smartlead publishes a dedicated help article for Outlook tenant error 550 5.7.708. When your sequencer writes a knowledge base article for your failure mode, the failure mode is routine. Our head-to-heads: Mission Inbox vs HyperTide and Mission Inbox vs Outlook.
The Pax8 incident
In March 2026 this model produced its clearest cautionary tale, and our co-founder wrote about it publicly at the time:
“They buy Microsoft mailboxes through Pax8. Then they spin up 25 to 99 inboxes per domain on Azure. Then they package that up as a ‘service’ for cold email teams... They didn’t stop at selling to end users. They sold to resellers. Those resellers sold to more resellers. A reseller of resellers of a reseller.... Then the music stops. Upstream notices abuse. Licenses get pulled. Tenants get flagged. Overnight, a lot of users woke up to dead mailboxes, broken warm-up pools, and campaigns bouncing.”
Two honest caveats. Pax8 is not the villain and we want to be explicit: they are a legitimate distributor with real licences, and the problem was the arbitrage layer built on top of them. And this is a first-hand practitioner account, not documented news. Neither Microsoft nor Pax8 published anything, so treat it as what we watched happen to customers.
What is documented is the direction. From January 2026 Microsoft requires indirect CSP resellers to pass business vetting and meet security requirements. And there is a public case of a Pax8 indirect reseller losing CSP status with the relevant line being: “Neither party is required to offer an explanation for the decision to terminate the partner agreement.”
The ceiling on stage 2
Tenant blocks that take 25 mailboxes at once. TERRL caps that tighten as you grow. A licensing arrangement Microsoft has publicly committed to closing. Stage two is not a destination, it is a cheaper way to rent the same landlord’s building with more roommates.
Stage 3: dedicated SMTP, and what separates the real ones
SMTP providers run their own mail servers instead of reselling Google or Microsoft. That removes upstream suspension risk entirely. Nobody can pull your licences, because there are no licences. Read our primer on SMTP relay services if the category is new to you.
It also changes where reputation comes from, and this is where most SMTP vendors fail.
| Maildoso / Mailforge | Mailreef | Infraforge | Mission Inbox |
|---|
| IP model | Shared pool | Dedicated server + IP | Private server, IP is a $99/mo add-on | Dedicated, pre-warmed 2 to 3 months |
| Arrives warm? | Inherits pool history, good and bad | No | No | Yes |
| Volume per IP | Pooled | New IP required roughly every 50,000 sends/mo | Volume-sensitive | No per-IP purchase tier |
| If the IP is blacklisted | You inherit it | Rebuild, per their own support answer | Restart the reputation ramp | Automatic rotation to a clean IP |
| Entry price | ~$2.40 to $3/mailbox/mo | $240 to $249/mo per server | $4/mailbox/mo + $99/IP | $50/mo, dedicated IPs included |
| Domain ownership | Vendor sells them | Bring your own | Vendor sells them | You buy and own them |
The honest trade, and the honest answer to it
Remember the dilution argument. On Google, a crowd of legitimate mail carries your reputation. An SMTP server has no such crowd. Nobody uses a cold-email SMTP provider for day-to-day correspondence. The only traffic is prospecting, which is the category of mail most likely to be marked as spam.
So an SMTP provider’s IP reputation is not inherited. It has to be manufactured. And that is the entire dividing line in stage three.
Manufacturing IP reputation means buying IPs, registering them in your own name, and sending warm-up traffic through them for months while they earn nothing. It is a margin problem, not a technology problem, and it is precisely what a vendor competing on being the cheapest cannot afford to do.
That is why “dedicated IP” on a feature checklist is close to meaningless. A brand new dedicated IP is a blank slate, and blank is not clean. It is unknown, and unknown is a soft negative when you are sending cold outbound.
Look at how Mailreef handles it. They tier you: past roughly 50,000 sends a month you must buy another IP. That is not arbitrary, it is what you do when the IP has no reputation and more volume would get it listed. Ask what happens if your dedicated IP gets blacklisted and the answer is essentially: start from scratch, buy new domains, do it again. See Mission Inbox vs Mailreef.
Infraforge has the same problem from the other direction. Their marketing says each mailbox gets a dedicated IP; their pricing page sells IPs at $99 a month each on top of $4 per mailbox. Both cannot be true. Twenty-five mailboxes is about $83 a month; twenty-five genuinely dedicated IPs would be $2,475 at their own rate.
And Maildoso and Mailforge do not attempt it at all. They run shared pools, which puts you back in a crowd, except this crowd is sending nothing but cold email. Full detail in our full Mailforge breakdown and Mission Inbox vs Maildoso.
What Mission Inbox does
We buy IPs, register them under our own name, and warm them for two to three months before they touch a customer account. During that window they send warm-up traffic only. Once assigned, we keep warming alongside your sending. Run a WHOIS on the IP your mail leaves from and it points to us.
That is why we cost more, and we would rather say it than dance around it. Carrying an IP for three months while it earns nothing is expensive. It is also the only way to hand someone an IP that is both exclusively theirs and already trusted.
The rule underneath all of this is worth memorising regardless of vendor:
You cannot start sending from a brand new IP with brand new domains. One of the two has to already have a reputation. Either bring aged domains with history onto a cold IP to warm it, or put new domains on an already-warm IP. New plus new is the most common self-inflicted deliverability failure there is. Our domain warming guide covers the ramp in detail.
The other difference is what happens when something does get listed, because eventually something does. Our monitoring checks every IP against the blacklists continuously. If one gets flagged we pull it from your pool automatically, assign a different dedicated IP, and sending continues. No pause, no rebuild, no new domains. That plus workload isolation is what Cubes and MI Shield exist to do.
And the things we are bad at
- We are not a sequencer. We sit underneath Instantly, Smartlead, EmailBison, Apollo, Salesloft, Outreach or your CRM.
- We are the expensive option. Customers tell us so directly. We argue about what you are buying, not about the price.
- We do not sell domains. Deliberately. You buy them, you own them, they stay yours if you leave.
- Setup takes about three weeks, not three days. We would rather lose a deal than compress the warm-up.
- We are wrong for stage 0 and stage 1. If you have not proven the offer, do not buy us.
The rule that decides which stage you are in
Infrastructure is a multiplier, not a fix.
If you are running a sub-1% reply rate, moving to dedicated infrastructure will not save you. A multiplier applied to a weak number produces a slightly different weak number and you will have paid more for it. That is a copy, offer or list problem, and we will tell you so on a sales call before we take your money.
If you are getting good results on rented rails and you want to scale past what they safely allow, or you want to stop being exposed to someone else’s policy decisions, that is when stage three earns its price. You are buying headroom and ownership, not a rescue. Worth reading delivery is not deliverability before you benchmark anything.
So what should you actually buy
| Where you are | What to do | Why |
|---|
| Stage 0. No proven offer, reply rate under 1% | Do not buy infrastructure. Fix the offer, the list and the copy | Infrastructure multiplies a result. A multiplier on a bad number is still a bad number. |
| Stage 1. First campaigns, under 20,000/mo, testing whether outbound works | A Google reseller. Run the panel check first | Cheapest way to find out. Just know what you are renting and from whom. |
| Stage 2. Volume on a budget, Microsoft-heavy ICP, can absorb downtime | Azure tenants, eyes open | Cheapest per mailbox at volume. You are accepting tenant blocks and routine disconnections as a cost. |
| Stage 3. Getting real replies, need to scale past what rented rails allow | Dedicated, pre-warmed SMTP | You have proof the offer works. Now buy rails that cannot be switched off by someone else. |
| Stage 3. Already hit a suspension, tenant block or blacklist | Mission Inbox | What happened to you was structural, not bad luck. Owning the reputation surface is the only fix. |
| Stage 3+. Enterprise ICP behind Proofpoint, Mimecast or Barracuda | Mission Inbox + managed warm-up | Security gateways weight IP reputation and domain age heavily. Sequencer warm-up is not enough. |
How to check any of this yourself
- Ask for admin console access and a billing relationship in your own name. If a Google or Microsoft vendor cannot give you both, you are on somebody else’s tenant. Full walkthrough: how to check whether your mailboxes are panel accounts.
- Send yourself a test email and read the raw headers. The sending IP is right there. Look up who owns it.
- Ask how long the IP has been warming and what it has been sending. Vagueness is the answer.
- Ask what happens when an IP or tenant gets blocked. “We rotate you automatically” and “you rebuild from scratch” are very different products.
- Check your own domain reputation first so you know what you are comparing against. Here is domain reputation check.
- Read Trustpilot sorted lowest-first and compare it to G2. A large gap usually means solicited onboarding reviews on one side and month-three reviews on the other.
If you want the condensed version of everything above, grab the deliverability cheat sheet.
Don’t take our word for it
“Super happy with the results. I’m seeing a 33% reply rate with over 54% positive replies on one campaign, and 74% open rates with zero bounces.” — Sary Jalkh, Growth Lead
“Mission Inbox deliverability is now neck and neck with G-suite and Microsoft infra, and in our case, 2x better.” — Ali Syed, Founder
“Our deliverability has improved significantly with Mission Inbox, which resulted in a 7% reply rate on our latest B2B campaign, compared to the average 3% rate.” — Theo, G2 review, April 2026
More on our testimonials page, plus the Practical Prospecting and Unit21 case studies.
Frequently asked questions
What is the best cold email infrastructure in 2026?
There is no single best, because these are three stages rather than three competitors. Google Workspace resellers suit stage one, under roughly 20,000 emails a month. Azure tenant providers are cheapest at volume if you can absorb tenant blocks. Dedicated, pre-warmed SMTP is where teams end up once rented rails cap out. Pick the stage you are in first.
Are cold email mailbox providers resellers?
Every vendor selling Google Workspace or Microsoft 365 mailboxes is a reseller, because only Google and Microsoft can provision those products. Premium Inboxes states this in their own FAQ. SMTP providers running their own mail servers are not, though some white-label a third-party platform, so check the headers.
What is an Azure tenant mailbox and is it safe?
A provider creates an Azure or Entra tenant, attaches a domain, and provisions 25 to 100 mailboxes on it. It is cheap per mailbox. The risks are Microsoft unusual-activity tenant blocks, routine disconnections, and tenant-level caps that scale sub-linearly. ScaledMail, itself an Azure provider, publicly warns that higher density raises tenant-block risk.
What is TERRL?
The Tenant External Recipient Rate Limit, rolled out by Microsoft through March and April 2026. It caps total external recipients per tenant per day, scaling sub-linearly: 1 licence allows 10,000, 100 licences allow 22,059, not 1,000,000. Breaching it blocks external sending for every user in the tenant.
Why did Google suspend cold email mailboxes in 2025?
Google began permanently suspending non-compliant workspaces in October 2025, targeting education, nonprofit and legacy free G Suite licences resold through panels for commercial outbound. Those licence types prohibit commercial resale, which is why suspensions arrived without the appeal path a direct customer would get. Notices were still going out in March 2026.
Is SMTP better than Google or Microsoft for cold email?
Only if the SMTP provider warms its own IPs. Google and Microsoft have good baseline deliverability because legitimate business email dilutes the cold outbound flowing through them. An SMTP server has no such crowd, so its reputation has to be manufactured through months of warm-up. A provider that does that beats rented rails. A provider that does not is worse than both.
Will switching to SMTP fix my low reply rate?
No. Infrastructure is a multiplier on a result, not a substitute for one. Under a 1% reply rate the problem is almost certainly the offer, the list or the copy.
Can I start sending from a new IP with new domains?
No. One of the two has to already have a reputation. Either bring aged domains with existing history onto a new IP, or put new domains on an already-warm IP. New plus new is the most common self-inflicted deliverability failure.
Try it for $50
If you are on rented rails today and getting real replies, the cheapest way to find out whether dedicated infrastructure moves your numbers is to run both at once. Keep your current setup, put a slice of the same campaign on Mission Inbox Lite at $50 a month, same copy, same list, same volume, and compare replies after three weeks.
That is how most of our larger accounts started.
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