553 5.1.8 Sender address rejected: Domain not found
The receiving server looked up the domain in your envelope sender address and found nothing, so it refuses to accept mail it could never bounce back to you. Postfix sends the temporary 450 4.1.8 form by default and the permanent 553 5.1.8 form when configured to; both mean the same thing about your sender domain.
What the server replies
450 4.1.8 <noreply@app.example.com>: Sender address rejected: Domain not found
- Sent by
- Any mail server
- Type
- PermanentRetry fails
- About
- Receiver policy
- Enhanced code
- 5.1.8
Who sends it
Postfix with reject_unknown_sender_domain (on by default in most distributions), Exim with verify = sender, and most gateways. It fires at MAIL FROM, before the recipient is even named.
Why it happens
- An application sends from a made-up domain such as noreply@localhost or noreply@app.internal.
- The bounce address uses a subdomain that has no MX or A record.
- The domain exists but its DNS is broken or its nameservers are unreachable at the moment of the check.
- A typo in the configured From or Return-Path.
How to fix it
Use a real domain in the envelope sender
The Return-Path domain must resolve. Set it to your main domain or a subdomain that has at least an A or MX record.
Give bounce subdomains an MX
If you use bounces.example.com for VERP, publish an MX for it (pointing at a server that accepts bounces) so it passes the check and so you actually receive the bounces.
Check DNS from outside
Run the sender domain through a DNS checker. If it fails to resolve from public resolvers, the receiving servers cannot see it either.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 550 5.1.2 Host unknown / bad destination system addressThe domain part of the recipient address is the problem: it does not resolve, so no server can be found to deliver to.
- 550 5.7.25 The IP address sending this message does not have a PTR record setupGmail looked up the reverse DNS of the connecting IP and either found nothing or found a hostname that does not resolve back to that IP.
- 501 5.5.4 Invalid HELO/EHLO argument / syntax error in parametersThe server did not like the arguments your client sent with a command, most often the hostname in HELO or EHLO.
Sources
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Other free tools
All tools- Email DNS checkerCheck MX, SPF, DKIM, DMARC and reverse DNS for any domain and get the exact records to add or fix.
- DMARC checkerLook up a domain's DMARC record, read every tag in plain words and see what to tighten next.
- DMARC record generatorBuild a valid DMARC record from a few choices and copy the host and value into your DNS.
- DMARC report analyzerDrop in a DMARC aggregate report and see who is sending as your domain and whether they pass.
- SMTP connection testerConnect to any SMTP server, check TLS and the login step, and see each stage with its timing.
- Email header analyzerPaste raw headers to see every hop, where the delay was, and whether SPF, DKIM and DMARC passed.
- Email address validatorCheck whether an address is well formed, whether its domain accepts mail, and whether it is disposable or a role account.