Microsoft 365 mailboxes, managed as sending infrastructure.
Connect Outlook or Microsoft 365 SMTP endpoints, respect Microsoft's per-user sending limits, and operate mailbox sending with the same discipline as ESP-backed routes.
- SMTP AUTH
- Microsoft 365SMTP AUTH
- Quotas
- Per-mailboxQuotas
- Reporting
- UnifiedReporting
- With ESPs
- PooledWith ESPs
Mailbox-based sending only works when it plays by the rules.
Outlook and Microsoft 365 permit authenticated SMTP relay under documented per-user limits. Those limits exist to protect the shared reputation of the Microsoft sending infrastructure.
Mailers.io wraps those mailboxes with platform-level orchestration — per-mailbox quotas, suppression, reporting, and clean failover into ESP pools — while staying inside Microsoft policy. We do not support or enable usage that breaks Microsoft's acceptable use.
From individual inboxes to a governed sending pool.
- Step 1
Attach mailbox credentials
Provide SMTP host, port, and authenticated credentials. Each mailbox attaches as a separate sending server.
- Step 2
Set realistic quotas
Cap each mailbox within Microsoft's documented per-user limits. We never exceed your configured cap.
- Step 3
Route & observe
Use mailbox pools for relationship-style sends. Fall through to ESP pools when mailbox limits are reached.
Platform-grade controls around Outlook sending.
Per-mailbox isolation
Every mailbox is its own sending server. Quota, reporting, and auth are scoped per inbox.
Quota enforcement
We respect per-user sending limits and will not push past the cap you set.
Pool with ESP routes
Combine mailbox sending with ESP-backed routes so volume scales cleanly beyond mailbox caps.
Suppression & templates
Unsubscribe, DNC, and template systems apply uniformly across mailbox and ESP sends.
Unified reporting
Mailbox sends appear in the same timeline as ESP sends, with provider attribution and response codes.
Policy-safe defaults
We do not bypass Microsoft policy. Accounts that look like TOS violations are throttled and flagged.
- Relationship-style outbound from Outlook mailboxes
- Teams that want mailbox sending operated with real controls
- Hybrid use cases pooling mailboxes with ESPs for scale
- Sending volumes that fit comfortably within Microsoft limits
- Per-user sending limits come from Microsoft — we do not exceed them
- High-volume cold or marketing email belongs on ESPs, not mailboxes
- Modern authentication (Basic Auth is deprecated) must be configured per Microsoft guidance
- Misuse of mailboxes violates Microsoft TOS and is not supported
Questions answered clearly.
Run Outlook sending like infrastructure. Stay inside Microsoft policy.
Connect Microsoft 365 mailboxes, pool them with ESPs, and operate with real quotas, suppression, and reporting.