5 Best Email Infrastructure Providers for Cold Outreach

InboxKit is the most complete email infrastructure provider we reviewed. It ships three tiers on annual billing: Professional at $31/mo includes 10 mailbox slots (extra mailboxes at $3.10 each), Agency at $81/mo includes 30 mailbox slots (extra at $2.70 each), and Enterprise at $250/mo includes 100 mailbox slots (extra at $2.50 each). Volume discounts are available for 500+ mailbox configurations.

The effective per-mailbox cost falls as you scale, with $2.50/mailbox being the Enterprise-tier rate, not a universal price. Across every tier you get Google Workspace, Microsoft 365, and Azure mailboxes with US-based IPs. That flexibility matters because some domains perform better on Google and others on Microsoft, and InboxKit lets you mix both under one roof.

What sets InboxKit apart from every other provider on this list is the deliverability stack built into the platform. Every tier includes automated DNS setup (SPF, DKIM, DMARC, MX records), inbox placement testing, an email verifier, a blacklist checker, real-time monitoring, unlimited team members, full API access, and the official admin panel with 2FA and app passwords. Email Warmup is an add-on at $3/mailbox/month across all tiers.

Azure tenant provisioning is $30 per domain. InfraGuard is the protection suite with blacklist, DNS, and "monster" monitoring plus unlimited placement tests — the first month is free. InboxKit also provides a dedicated panel per domain, which makes it easy to monitor health at a granular level.

The platform supports 24+ sequencer integrations including SendKit, Instantly, Smartlead, and Saleshandy. The SendKit integration is especially strong: one-click import with tag and workspace selection, fully automated via API key with no manual login required. The provider claims a 95% inbox placement rate.

Trusted by companies like 11x, Zendesk, PwC, and Merck. For teams that want infrastructure plus the deliverability tooling layer in one provider, InboxKit is the clear leader at the Agency and Enterprise tiers.

Maildoso is the volume leader in email infrastructure. They manage over 400,000 mailboxes and handle 10+ million emails per day. If you need to spin up 100 or more SMTP mailboxes quickly and affordably, Maildoso is the most proven option at scale.

SMTP mailboxes start at $1.90 to $2.50 per mailbox per month. Combo mailboxes (SMTP plus Google Workspace) run $2 to $3. Domains are available from $2 per domain, which helps keep total infrastructure costs low.

The platform offers auto DNS configuration, one-click integrations with Instantly, Smartlead, and Saleshandy, and both API and MCP support for automation. Maildoso carries a 4.7 rating on G2 from 159 reviews, which is the strongest social proof of any provider on this list. The platform is battle-tested at scale.

The trade-off is that Maildoso focuses on mailbox provisioning and DNS. It does not include built-in warmup, inbox placement testing, or an email verifier. You will need your sending tool (like SendKit) or a standalone warmup service to handle deliverability.

For teams that already have a full sending platform, this is not a problem. For teams that want everything in one place, InboxKit offers more.

Zapmail focuses exclusively on Google Workspace mailboxes, which gives it an advantage for teams that prefer Google deliverability over SMTP. Their pricing follows a tiered model: Starter at $39/mo for 10 mailboxes ($3.50 per extra), Growth at $99/mo for 30 mailboxes ($3.25 per extra), and Pro at $299/mo for 100 mailboxes ($3.00 per extra). The standout features are their AI tools.

The AI domain name generator helps you pick domains that look natural and avoid spam triggers. The AI mailbox namer creates realistic sender profiles. Pre-warmed accounts mean you can start sending sooner.

Workspace-level isolation keeps each domain separate. Zapmail has set up over 1 million mailboxes and manages 330,000+ domains, which proves the platform works at scale. They hold a 4.5 TrustPilot rating.

The downside is cost. At $3.00 to $3.90 per mailbox, Zapmail is more expensive than SMTP providers like Maildoso ($1.90) or Mailforge ($2.00). Google Workspace mailboxes do tend to have better inbox placement than SMTP, but the per-mailbox cost difference adds up at scale.

No Microsoft 365 or Azure option either.

Mailforge is the shared-IP SMTP provider from Salesforge. Mailboxes cost $2 to $3 per mailbox per month on shared IP infrastructure. The platform includes automated DNS setup, bulk DNS updates, domain transferring, SSL and domain masking, and support for multiple workspaces.

The cost comparison is compelling: 200 mailboxes on Mailforge costs roughly $484 per month, versus $1,680 for the equivalent on Google Workspace. That is a 71% savings. Over 10,000 businesses use Mailforge, making it a well-established option in the SMTP infrastructure space.

Mailforge integrates with the broader Salesforge ecosystem, which includes Infraforge (dedicated IPs) and the Salesforge AI sales engagement platform. If you are already in that ecosystem, Mailforge fits naturally. The limitation is shared IPs.

Every mailbox on Mailforge sends through shared IP pools, which means your deliverability depends partly on the behavior of other senders on the same IPs. For teams that need IP isolation, Infraforge (the dedicated IP option) or InboxKit are better choices.

Infraforge is the dedicated-IP counterpart to Mailforge, also from the Salesforge ecosystem. Mailboxes cost $3 to $4 per mailbox per month with dedicated IP infrastructure. The platform includes pre-warmed domains, multi IP provisioning, a masterbox for centralized management, and API access.

The dedicated IP pricing adds $99 per month on top of per-mailbox costs. SSL and domain masking costs $2 per domain per month. These add-ons push the total cost higher than shared IP providers, but the trade-off is meaningful: your sender reputation is isolated from other users.

Infraforge makes the most sense for teams that need SMTP mailboxes with IP isolation but do not want to use Google Workspace or Microsoft 365. The pre-warmed domains reduce ramp-up time. The downside is total cost.

A setup with 100 mailboxes at $3.50 average plus $99 for dedicated IP plus domain masking fees adds up faster than InboxKit at the Enterprise tier ($250/mo for 100 mailbox slots, effective $2.50 per mailbox plus the warmup add-on if needed) with Google Workspace and most deliverability tools bundled. For teams already in the Salesforge ecosystem, Infraforge is a natural upgrade from Mailforge.