5.1.2 bounce: bad destination system address
5.1.2Bad destination system addressPermanent: retrying will not help
550 5.1.2 <user@example.invalid>: Host unknown, domain does not exist
What it means
X.1.2 is "Bad destination system address" in RFC 3463. The problem is the domain or host part of the recipient address, not the mailbox name. Either the domain does not exist in DNS, or the server you handed the message to does not recognise it as a destination it can deliver to.
Two different machines produce it. Your own outgoing server can generate a 5.1.2 DSN when it finds that the domain has no MX and no address record. A relay or smart host can return 550 5.1.2 when it refuses a domain it is not configured to route. Exchange uses 5.1.2 in some "host unknown" style bounces, and many relays use it when a domain is misspelled.
A 4.1.2 means the domain could not be checked right now, often a DNS timeout, and the message stays queued. Compare it with 5.4.4 (the domain exists but no route to a mail server could be built) and 5.1.1 (the domain is fine, the mailbox is not).
Likely causes
- A typo in the domain, such as gmial.com or example.con.
- The domain has expired, been deleted, or was never registered.
- The domain exists but has no MX record and no A or AAAA record to fall back on.
- Your relay or smart host has a restricted list of destination domains and this one is not on it.
How to fix it
- Read the domain in the bounce carefully and fix any spelling mistake in the address.
- Look up the domain’s MX records. If there are none and no A record either, the domain cannot receive mail and you need another address for the person.
- Check whois for the domain. An expired registration explains a sudden 5.1.2 for an address that used to work.
- If you own the domain and people report this bounce, restore the MX records at your DNS provider, for example `example.com. 3600 IN MX 10 mail.example.com.`, and confirm the zone is served by working nameservers.
- If the rejection came from your own relay, add the destination to its routing table or allow outbound mail to any domain.
Checks that apply
- MX Lookup Shows whether the domain publishes MX records at all, the first thing a 5.1.2 calls into question.
- Whois Lookup Reveals an expired or unregistered domain, which turns every address at it into a 5.1.2.
- DNS Server Audit Audits the domain’s nameservers, so you can see a broken delegation that makes the domain look nonexistent.
- Typosquat Scan Suggests the likely intended domain when the one in the bounce is a misspelling.
FAQ
- Is 5.1.2 the same as 5.1.1?
- No. A 5.1.2 points at the domain or host part of the address, while 5.1.1 means the domain is fine but the mailbox name is unknown.
- The domain works in a browser, so why 5.1.2?
- A website only needs an A record. Mail needs an MX record or a usable fallback address, and some servers reject a domain with a web host but no mail host.