Learn why SMTP rotation improves cold email deliverability and how to set up multi-inbox sending. Step-by-step guide with provider recommendations.
SMTP rotation means sending your cold emails through multiple mailboxes instead of just one. Instead of sending 100 emails from one inbox, you send 33 from inbox A, 33 from inbox B, and 34 from inbox C.
Email providers track sending volume per mailbox. High volume from a single mailbox triggers spam filters. Rotation solves this:
| Metric | Single mailbox | 3 mailboxes rotated |
|---|---|---|
| Daily volume | 50 safe max | 150 safe total |
| Risk per mailbox | High | Low |
| If one gets throttled | All sends stop | 2 others continue |
| Warm-up time | 4 weeks to 50/day | 4 weeks to 150/day |
| Provider diversity | None | Can mix Gmail + Outlook |
Cost: ~$10-15 per domain per year.
Test each with mail-tester.com — all must score 9+/10.
In HeraMail, go to Settings → SMTP and add each mailbox. When you dispatch a campaign, HeraMail automatically rotates through your connected mailboxes using round-robin distribution.
| Provider | Price | Sending limit | Best for |
|---|---|---|---|
| Google Workspace | $6/user/mo | 2,000/day | Reliability |
| Microsoft 365 | $6/user/mo | 10,000/day | Volume |
| Zoho Mail | $1/user/mo | 250/day | Budget |
| OVH | $2/mailbox/mo | 200/day | Europe/GDPR |
| Amazon SES | Pay per email | 50,000/day | High volume |
HeraMail supports multi-SMTP rotation natively — add your mailboxes and we handle the round-robin: heramail.io