Sending & Deliverability
Understanding Email Bounce Types
Hard vs Soft Bounces
A hard bounce is a permanent delivery failure. The recipient address does not exist, the domain is invalid, or the receiving server has explicitly rejected the message. AcelleMail automatically unsubscribes hard-bounced addresses to prevent further damage to your sender reputation.
A soft bounce is a temporary failure. The mailbox may be full, the server temporarily unavailable, or the message too large. AcelleMail retries soft bounces according to your queue retry settings before eventually marking them as failed.
SMTP Bounce Codes
| Code | Type | Meaning |
|---|---|---|
| 550 | Hard | Mailbox does not exist |
| 551 | Hard | User not local |
| 553 | Hard | Mailbox name invalid |
| 421 | Soft | Service temporarily unavailable |
| 452 | Soft | Mailbox full |
| 451 | Soft | Local processing error |
How AcelleMail Processes Bounces
AcelleMail uses two methods to detect bounces:
- Feedback Loop (FBL) — ISPs send complaint notifications back to your sending server.
- Bounce mailbox polling — AcelleMail connects to a dedicated bounce email inbox (IMAP/POP3) and parses incoming DSN (Delivery Status Notification) messages.
Configure your bounce mailbox under Settings → Sending Servers → [your server] → Bounce Handler.
Best Practices
- Keep your hard bounce rate below 2% — most ISPs throttle senders who exceed this.
- Run a list verification pass before every large campaign.
- Never re-add a hard-bounced address manually without explicit re-confirmation from the subscriber.
- Review your bounce log monthly: Reports → Bounced Subscribers.