5.2.1 bounce: mailbox disabled, not accepting mail
5.2.1Mailbox disabled, not accepting messagesPermanent: retrying will not help
550 5.2.1 <user@example.com>: Mailbox disabled, not accepting messages
What it means
X.2.1 is "Mailbox disabled, not accepting messages" in RFC 3463. The receiving server recognises the address and admits a mailbox is attached to it, but the mailbox has been switched off. That separates it from 5.1.1, where the address does not exist at all.
Consumer providers use it for suspended, inactive, or deleted-but-retained accounts. Gmail returns 550 5.2.1 for disabled accounts, and other consumer services word similar replies their own way. Corporate systems use it when an account is blocked during offboarding, a legal hold, or a security lockout. Gmail also uses 4.2.1 when a single user is receiving mail too fast, which is a different meaning on its own page.
RFC 3463 notes that X.2.1 can be either temporary or permanent. A 4.2.1 means "not right now" and your server retries. A 5.2.1 is final: mailing list senders should treat it like an unknown user and remove the address.
Likely causes
- The mailbox owner left the organisation and the account was disabled rather than deleted.
- A free mail account was suspended for inactivity, a terms of service breach, or a security issue.
- An admin blocked mail delivery to the mailbox on purpose, for example during an investigation.
- The account was disabled by its provider after it was compromised and used to send spam.
How to fix it
- Confirm the person’s current address through another channel; a disabled mailbox will not come back on its own in most cases.
- Remove or suppress the address in your mailing tools after one 5.2.1, since repeated sends to inactive accounts damage your sender reputation.
- If you are the recipient’s admin and the block was not intended, re-enable mail delivery for the account and check whether a retention or offboarding policy disabled it.
- If a user reports that they cannot receive mail and their provider is Gmail or Yahoo, they should sign in and follow the provider’s account recovery prompts.
Checks that apply
- MX Lookup Identifies the mailbox provider, which tells you whether the disable is a consumer suspension or a company policy.
- SMTP Diagnostics Shows the receiving server’s banner and software, useful when you report the problem to the recipient’s IT team.
FAQ
- Is a 5.2.1 bounce permanent?
- The 5.2.1 form is. If the recipient fixes their account, mail will flow again, but you cannot speed that up from your side.
- How is 5.2.1 different from 5.1.1?
- 5.1.1 says the address is unknown. 5.2.1 says the address exists but the mailbox is switched off.