X.2.0 · Mailbox
SMTP enhanced status code 5.2.0
Other or undefined mailbox status
X.2.0 says the mailbox exists but something about it caused the failure, without naming what. Servers use it for mailbox states that do not fit the more specific codes.
| Registry code | X.2.0 |
|---|---|
| Forms in use | 5.2.0, 4.2.0 |
| Registry name | Other or undefined mailbox status |
| Subject | 2: Mailbox |
| 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 mailbox exists, but something about the destination mailbox has caused the sending of this DSN.
IANA SMTP Enhanced Status Codes registry, X.2.0
In practice
X.2.0 is a catch-all for mailbox problems that are not full, disabled or too-large cases. Servers use it for conditions such as a mailbox locked during migration, or a per-user rule rejecting the message. Because the code is vague, the free text matters more than the number. If an address returns 5.2.0 on several sends weeks apart, treat it as dead for list purposes even if the server never says so. Because it says the mailbox exists, X.2.0 is not a reason to delete an address after a single bounce. It is a reason to watch the address.
What the sender should do
Read the text after the code. If it repeats for the same address over several sends, suppress the address.
Would verifying the list have prevented 5.2.0?
Partly
The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Common questions
- What does 5.2.0 mean?
- X.2.0 says the mailbox exists but something about it caused the failure, without naming what. Servers use it for mailbox states that do not fit the more specific codes.
- What is the difference between 5.2.0 and 4.2.0?
- They share the subject and detail digits, so they report the same condition, "Other or undefined mailbox status". 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 5.2.0?
- Read the text after the code. If it repeats for the same address over several sends, suppress the address.
- Would an email verifier have caught 5.2.0?
- Partly. The reply is not only about one address, but providers weigh how often a sender hits addresses that do not exist. Removing bad addresses before sending lowers that signal.
Sources
- IANA: SMTP Enhanced Status Codes registry (checked 2026-11-05)
- RFC 3463: Enhanced Mail System Status 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