X.1.2 · Addressing
SMTP enhanced status code 5.1.2
Bad destination system address
X.1.2 means the domain after the @ does not exist or cannot receive mail, usually because it has no MX or A record. Every address on that domain fails, so it often points to a typo such as gmial.com or to a domain that has lapsed.
| Registry code | X.1.2 |
|---|---|
| Forms in use | 5.1.2, 4.1.2 |
| Registry name | Bad destination system address |
| Subject | 1: Addressing |
| Permanent or temporary | 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed |
| Associated basic codes | Not given |
| Defined in | [RFC 3463] |
What the registry says
The destination system specified in the address does not exist or is incapable of accepting mail. For Internet mail names, this means the address portion to the right of the "@" is invalid for mail. This code is only useful for permanent failures.
IANA SMTP Enhanced Status Codes registry, X.1.2
In practice
X.1.2 points at the domain rather than the person. Typical triggers are typos of large providers (hotmial.com, gmal.com), company domains that have expired since the address was collected, and domains that exist for a website but never set up mail. Senders sometimes see it as 553 5.1.2 from Gmail when a trailing dot or space follows the address. Because every address on such a domain fails, one DNS lookup during verification clears a whole group of rows at once.
What the sender should do
Fix the domain if it is a typo of a known provider, otherwise remove the address. A verifier checks DNS for every domain in the list, so dead domains are flagged without sending anything.
Would verifying the list have prevented 5.1.2?
Yes, verification catches this
A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
How providers use X.1.2
Quoted from each provider’s own error reference.
- Gmail 553 5.1.2
We weren't able to find the recipient domain. Please check for any spelling errors and make sure you didn't enter any spaces, periods, or other punctuation after the recipient's email address. For more information, go to Fix bounced or rejected emails.
Common questions
- What does 5.1.2 mean?
- X.1.2 means the domain after the @ does not exist or cannot receive mail, usually because it has no MX or A record. Every address on that domain fails, so it often points to a typo such as gmial.com or to a domain that has lapsed.
- What is the difference between 5.1.2 and 4.1.2?
- They share the subject and detail digits, so they report the same condition, "Bad destination system address". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.
- What should I do about 5.1.2?
- Fix the domain if it is a typo of a known provider, otherwise remove the address. A verifier checks DNS for every domain in the list, so dead domains are flagged without sending anything.
- Would an email verifier have caught 5.1.2?
- Yes, verification catches this. A mailbox check asks the recipient's server whether the address and domain can receive mail, without sending anything, so the address is flagged before the send instead of bouncing against your domain.
Full guides for this code
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status Codes (checked 2026-11-05)
- Google Workspace Admin Help: Gmail SMTP errors and codes (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05