450 4.1.8 <user@example.com>: Sender address rejected: Domain not found
The sender's domain does not resolve in DNS
Seen in Postfix (reject_unknown_sender_domain), Sendmail (553 5.1.8)
The receiving server looked up the domain in the envelope sender (MAIL FROM) address and found no MX, A or AAAA record, so it refused the mail.
Common causes
- The app sends from a made-up or internal domain such as example.local or server.lan.
- A typo in the sender's domain.
- The sender's domain expired or its DNS is broken (Postfix uses 450 for temporary DNS failures).
How to fix it
- Send from a real domain you own; set myorigin or the app's From and Return-Path accordingly.
- Make sure the sender domain has an MX or A record.
- Check the domain registration has not lapsed.
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- 421 4.7.0 Try again later · Temporarily deferred by the receiving server
- 550 5.7.515 Access denied, sending domain does not meet the required authentication level · Outlook.com rejected unauthenticated bulk mail
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