SMTP error 533
Non-standard label, usually a misread of 5.3.3.
Updated Jul 1, 2026
SMTP 533 is not a valid reply code. RFC 5321 defines no 533. The closest authoritative match is enhanced status 5.3.3, "system not capable of selected features", meaning the destination cannot support a feature the message requested. The old "out of disk space" explanation is wrong; storage problems map to 452 or 552. Read the server's full reply text.
Read the full server text; if it is 5.3.3, stop requesting the unsupported SMTP extension.
What it means
RFC 5321 does not define a 533 reply, and it is not a registered code. Its permanent codes are 500-504 and 550-555 (with 521 and 556 added later). So a reported "533" is almost always a misread of the enhanced status 5.3.3 written without the dots, or a vendor label that does not match anything the server sent.
Common causes
If the bounce actually contains 5.3.3, it means the receiving system rejected the message because it asked for an SMTP extension it cannot honor (for example 8BITMIME, SMTPUTF8, DSN, SIZE, or BINARYMIME, or a gateway feature). The leading 5 makes it permanent.
A "533" that matches nothing the server sent came from your sending application, not the receiver.
How to fix it
- Capture the full server reply
Do not act on the bare number. Read the whole line, which contains the real 3-digit code and often the dotted enhanced code plus text (for example "550 5.3.3 Feature not supported").
- If it is truly 5.3.3, stop requesting the unsupported extension
Check the EHLO response to see which extensions the destination advertises, then disable the offending option in your client (for example turn off SMTPUTF8 or 8BITMIME, or send 7-bit content) and retry.
- If the text mentions storage or mailbox full
That is a different code: 452 for a temporary shortage (retry later) or 552 for a permanent over-quota mailbox. Neither is 533.
- If the text mentions the address, relaying, or policy
Look for 550 (no such user), 553 (mailbox name not allowed), or 554 (policy). Correct the recipient address instead of treating it as a feature error.
- If no code matches, check your app
A "533" with no matching server code came from your sending integration. Check its documentation for its own numbering.
Paired code
A real bounce often shows both a 3-digit code and an enhanced code together. This one commonly pairs with:
Common questions
Is 533 a valid SMTP reply code?
No. RFC 5321 defines 500-504 and 550-555 (plus 521 and 556 later). There is no 533. It is usually a misformatted enhanced status 5.3.3 or a vendor label.
Does 533 mean the recipient server is out of disk space?
No. That old explanation is wrong. Storage problems are 452 (temporary) or 552 (permanent over-quota), not 533.
What does 5.3.3 mean?
The destination system cannot support a feature the message requested, such as an SMTP extension the receiver does not implement. It is permanent, so resending unchanged will not help.