When an email cannot be delivered, the sender usually receives an automatic bounce mail (also called a bounce back or non-delivery report) from the mail server – often from the "Mail Delivery System" or "MAILER-DAEMON".
These messages almost always include an SMTP status code (e.g., 550, 552) and a short description. There are two main categories:
Here are the most common messages that occur with mail.fr (and are similar on most other providers), including the original English text, meaning, and recommended action:
| SMTP Code | Original Message (English) | Meaning / Explanation | Type | What to do? |
|---|---|---|---|---|
| 550 5.1.1 | User unknown / No such mailbox / Unknown recipient | Recipient unknown / Address does not exist | Hard Bounce | Check the address, correct it, or remove it from your list |
| 550 5.1.2 | No such mailbox / Mailbox disabled | Mailbox no longer exists or has been disabled | Hard Bounce | Check the address or delete it |
| 552 5.2.2 | Mailbox full / Over quota / The recipient's inbox is out of storage space | Mailbox full / Storage quota exceeded | Soft Bounce | Try again in a few hours or days |
| 550 | Recipient address rejected: Access denied | Address rejected (e.g., blocklist, spam filter) | Hard Bounce | Contact the recipient another way; possibly incorrect blocklist |
| 550 | undeliverable address / unrouteable address | Address undeliverable / Domain not reachable | Hard Bounce | Check the address/domain (typo?) |
| 550 5.7.1 | Relay access denied / ... identified as spam | Relay not permitted / Identified as spam | Hard Bounce | Check sender reputation (rare for normal private users) |
| 451 / 452 | Temporary failure / Greylisted | Temporary error (Greylisting – anti-spam measure) | Soft Bounce | Delivery will be retried automatically after hours/days – do nothing |
| 550 | Mailbox unavailable / Account disabled | Mailbox temporarily unavailable or disabled | Usually Hard | Check the address, try again later |
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