Every cold email cost comparison stops at the per-mailbox price, which is the one number that does not decide the bill. Domains, DNS work, warmup and IPs are where the money hides, and one of the three viable routes is almost never priced at all.
Below are all three, at three scales, with the assumptions stated so you can argue with them.
The three routes
1. Private infrastructure on a provider’s ESP. Mailboundry sits here: $0.80 a mailbox on monitored shared pools, $1.30 on dedicated IPs, up to $2.50 at the top tier, billed monthly, quarterly or annually. SPF, DKIM, DMARC and MX are generated per domain automatically, and domains can be bought in-app up to $14 per TLD.
2. Real Google Workspace or Microsoft 365 mailboxes. $4 to $4.50 a mailbox a month depending on billing terms and reseller. Highest inherent sender trust available, pooled IPs with no dedicated option at any price, and a bill that scales linearly forever.
3. Self-hosted on Azure. Around $25 per domain per month for up to 50 mailboxes. Priced per domain rather than per seat, which changes the shape of the curve completely, and it is the route almost nobody writes about.
Assumptions
- Roughly 4 mailboxes per domain on routes 1 and 2, which is the conservative density most senders settle on.
- Domain registration at $14 a year, so about $1.17 a month per domain.
- Warmup included on route 1, and priced at zero on the others for simplicity, which flatters routes 2 and 3.
- No sequencer, data or AI costs, since those are identical whichever route you pick.
10 mailboxes
| Route | Mailboxes | Domains | Monthly total |
|---|---|---|---|
| Mailboundry Shared | $8 | 3 domains, about $4 | About $12 |
| Mailboundry Dedicated | $13 | About $4 | About $17 |
| Google / Microsoft | $40 | About $4 | About $44 |
| Azure, self-hosted | 1 domain at $25 | About $1 | About $26 |
At this scale the whole argument is worth about $30 a month, so pick on merit rather than price. Ten mailboxes is also well under the volume where a dedicated IP makes sense, which is the subject of the volume threshold.
The interesting line is Azure. Because it is priced per domain, it is already cheaper than Google at ten mailboxes and the gap only widens. The crossover against Google is around seven mailboxes.
100 mailboxes
| Route | Mailboxes | Domains | Monthly total |
|---|---|---|---|
| Mailboundry Shared | $80 | 25 domains, about $29 | About $109 |
| Mailboundry Dedicated | $130 | About $29 | About $159 |
| Google / Microsoft | $400 | About $29 | About $429 |
| Azure, self-hosted | 2 domains at $25 | About $2 | About $52 |
Two things happen here. The per-seat route is now four times the private-infrastructure route and eight times Azure, and the domain line has stopped being a rounding error: 25 domains is real registration spend and real DNS work, which is the point at which automatic per-domain authentication stops being a convenience and starts preventing silent failures.
Azure looks unbeatable on this table, and on price it is. What the table cannot show is that you now own a mail platform: the tenant, the authentication, the warmup discipline, the blocklist responses and the reputation. Priced at a few hours of engineering time a month, the gap against Mailboundry Shared closes to nothing.
500 mailboxes
| Route | Mailboxes | Domains | Monthly total |
|---|---|---|---|
| Mailboundry Shared | $400 | 125 domains, about $146 | About $546 |
| Mailboundry Dedicated | $650 | About $146 | About $796 |
| Google / Microsoft | $2,000 | About $146 | About $2,146 |
| Azure, self-hosted | 10 domains at $25 | About $146 | About $396 |
At 500 mailboxes the per-seat route costs about $19,000 a year more than private infrastructure on shared pools, and about $16,000 more than dedicated IPs. That is a headcount decision disguised as an infrastructure decision.
Note what happened to the Azure advantage: domains dominate. Once you need 100-plus domains, the domain line is the same on every route, and the per-domain pricing model stops being the differentiator it was at 100 mailboxes.
The crossover points, summarised
| Scale | Cheapest | What we would actually run |
|---|---|---|
| Under 10 | Private infra, marginally | Google or Microsoft. At this volume trust is worth more than $30 |
| 10–30 | Azure, then private infra | Private infra on shared pools, with a few Workspace mailboxes for named accounts |
| 30–100 | Azure | Private infra. Azure only if you have someone who wants to own it |
| 100–500 | Azure, then private infra | Private infra, dedicated IPs once volume per IP passes a few thousand a month |
| 500+ | Azure | Private infra with segmented dedicated IPs, one set per client for agencies |
The pattern: Azure wins on paper almost everywhere above ten mailboxes, and loses the moment you price the operational work honestly. Per-seat mailboxes are worth their premium only where inherent trust changes the outcome, which is enterprise and named accounts rather than volume prospecting.
The four costs that break comparisons
- Dedicated IPs as an add-on. $50 to $100 an IP a month is common. Ten IPs adds up to $1,000 that never appeared in the per-mailbox figure you compared.
- Domains. At 4 mailboxes per domain, 500 mailboxes means 125 domains. Always price the domain line explicitly.
- Billing terms. Some providers in this category have no monthly plan at all, so a volume that moves month to month costs you a year of commitment.
- Warmup and placement testing. Bundled on some routes, a separate subscription on others.
Next: whether your volume justifies a dedicated IP, or what the data layer costs in credits.
Frequently asked questions
What is the cheapest way to run cold email mailboxes?
Below about 10 mailboxes the differences are small enough not to matter. Above that, private infrastructure on shared pools is the cheapest per mailbox, from around $0.80, and self-hosting on Azure becomes competitive because it is priced per domain rather than per mailbox.
How much does Azure cost for cold email?
Around $25 per domain per month for up to 50 mailboxes, which works out under $1 a mailbox at full density. The trade is setup and operational work: you own the tenant, the authentication, the reputation and the ramp.
At what point does Google Workspace stop making sense?
Around 30 to 80 mailboxes, depending on how much you value inherent sender trust. Per-seat pricing is linear at $4 to $4.50 a mailbox, so the bill doubles every time the mailbox count does, while infrastructure pricing flattens.
Should I mix providers?
Most teams past 50 mailboxes should. Real Workspace or Microsoft mailboxes for named enterprise accounts where trust and reply threading matter most, private infrastructure for volume top-of-funnel.
What is missing from a per-mailbox price?
Domains, usually at up to $14 a year each, DNS management time, warmup tooling if it is not included, and dedicated IPs if they are priced as an add-on rather than bundled. Those four turn a lot of headline comparisons upside down.