550 5.1.2 Host unknown / bad destination system address
The domain part of the recipient address is the problem: it does not resolve, so no server can be found to deliver to. RFC 3463 calls 5.1.2 "bad destination system address". A receiving relay returns it when you hand it an address in a domain it cannot look up; your own MTA generates the same thing when it fails to route a message.
What the server replies
550 5.1.2 <info@example.invalid>: Recipient address rejected: Domain not found
- Sent by
- Any mail server
- Type
- PermanentRetry fails
- About
- Recipient problems
- Enhanced code
- 5.1.2
Who sends it
Postfix with reject_unknown_recipient_domain, Exchange ("550 5.1.2 Host unknown"), and sending MTAs that bounce a message after NXDOMAIN on the recipient domain. Microsoft 365 reports the same condition as 5.4.310.
Why it happens
- A typo in the domain.
- The domain expired and was dropped from DNS.
- The domain exists but has neither MX nor A records.
- An internal domain name that only resolves inside the recipient's network was used from outside.
How to fix it
Check the domain
Resolve it with a DNS checker. No MX and no A means there is nowhere to deliver; the recipient has to fix their DNS or give you a different address.
Correct the spelling
Compare it to a known-good address at the same company.
Admin: look at your resolver
If your server bounces mail to domains that do resolve elsewhere, its DNS resolver is misconfigured or blocked. Test with dig from the mail host itself.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 550 5.4.310 DNS domain does not existA Microsoft 365 user tried to send to a domain and Exchange Online could not find it in DNS: no MX record, no A record, nothing.
- 553 5.1.8 Sender address rejected: Domain not foundThe 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.
- 550 5.1.1 User unknown / recipient address rejectedThe most common bounce there is: the mailbox does not exist.
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.
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- 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.