In short: A bounce is the receiving or sending system's explanation of why a message was delayed or rejected. Keep the complete bounce: the SMTP code, diagnostic text and timestamps are far more useful than a screenshot saying only “failed”.
Temporary and permanent failures
- 4xx: normally temporary. The sending server may retry automatically.
- 5xx: normally permanent for that attempt. Something must change before resending.
The accompanying text is authoritative; the same code can be used for several policies.
Common explanations
- Unknown recipient: the address may be misspelled, removed or not configured.
- Mailbox full: the recipient has no available quota.
- Authentication failed: check the complete SMTP username, password and encryption settings.
- Blocked or rejected: the recipient may object to content, authentication, sending reputation or policy.
- Timed out: a network or remote-server problem may be temporary.
- SPF, DKIM or DMARC failure: the message did not authenticate or align as expected.
- Reverse-DNS or PTR complaint: this normally requires the operator of the sending IP address.
What to check
- Confirm whether sending works from webmail.
- Check that the From address is valid for the authenticated mailbox.
- Do not repeatedly resend while the cause is unknown.
- If only one recipient domain rejects mail, compare delivery to another independent provider.
What to send support
Provide the complete bounce, sender and recipient addresses, subject, send time with timezone and whether webmail behaves the same way. Redact unrelated message content, but keep SMTP status lines and message IDs. Never send the mailbox password.
