5.3.3: requested feature not supported
The message relies on a feature that the receiving system cannot provide, typically at a gateway that cannot carry it into another mail environment.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.3.3
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.3.3 System not capable of selected features
What it means
X.3.3 covers messages that request options the destination does not support. RFC 3463 gives the example of a gateway where a feature from one mail system has no equivalent in the other. It is rare, and usually tied to a gateway or to an optional SMTP extension the message depends on. RFC 3463 ties it to no class: 5.3.3 means the message was returned as it stands, while a 4.3.3 suggests the feature may become available later. Sending again without the special option usually gets the message through.
Common causes
- A gateway between mail systems that cannot carry a feature the message uses.
- A message that relies on an optional SMTP extension the next server does not have.
How to fix it
- 01Check which special options your sending software sets (delivery notices, priority, conversions) and resend without them.
- 02Read the text after the code, which may name the unsupported feature.
- 03If the message must keep that feature, ask the recipient's administrator whether another route supports it.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.3.5Receiving system misconfigured.The receiving system's own configuration does not allow it to accept this message, a problem only its administrator can fix.
- 5.6.1Content type not supported.The message uses a kind of content that the delivery protocol or the next system on the path cannot carry, so it was refused.
- 555Parameters not recognised.The server does not recognise, or does not implement, a parameter given with MAIL FROM or RCPT TO, so that command failed.
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