X.2.1 · Mailbox
SMTP enhanced status code 5.2.1
Mailbox disabled, not accepting messages
X.2.1 means the mailbox exists but is disabled and not accepting messages. As 5.2.1 it usually means an inactive or suspended account; as 4.2.1 some providers use it for a recipient receiving mail too fast.
| Registry code | X.2.1 |
|---|---|
| Forms in use | 5.2.1, 4.2.1 |
| Registry name | Mailbox disabled, not accepting messages |
| 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 is not accepting messages. This may be a permanent error if the mailbox will never be re-enabled or a transient error if the mailbox is only temporarily disabled.
IANA SMTP Enhanced Status Codes registry, X.2.1
In practice
Providers use X.2.1 in two very different ways. As 550 5.2.1, Gmail reports an account that is inactive, which is close to a hard bounce. As 450 4.2.1, Gmail reports a recipient receiving mail faster than it allows, which is a rate limit and clears with time. The class digit tells you which one you have: 5 means suppress the address, 4 means slow down and let the queue retry.
What the sender should do
For 5.2.1, remove the address or suppress it for a long period, because disabled accounts rarely come back. For 4.2.1, let your server retry later and slow the sending rate to that recipient.
Would verifying the list have prevented 5.2.1?
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.
How providers use X.2.1
Quoted from each provider’s own error reference.
- Gmail 450 4.2.1
The user you are trying to contact is receiving email too quickly. Please resend your message at a later time. If the user is able to receive email at that time, your message will be delivered. For more information, go to Limits for sending & getting email.
- Gmail 550 5.2.1
The email account that you tried to reach is inactive. For more information, go to https://support.google.com/mail/?p=DisabledUser
Common questions
- What does 5.2.1 mean?
- X.2.1 means the mailbox exists but is disabled and not accepting messages. As 5.2.1 it usually means an inactive or suspended account; as 4.2.1 some providers use it for a recipient receiving mail too fast.
- What is the difference between 5.2.1 and 4.2.1?
- They share the subject and detail digits, so they report the same condition, "Mailbox disabled, not accepting messages". 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 5.2.1?
- For 5.2.1, remove the address or suppress it for a long period, because disabled accounts rarely come back. For 4.2.1, let your server retry later and slow the sending rate to that recipient.
- Would an email verifier have caught 5.2.1?
- 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.
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