5.6.1 bounce: media not supported
5.6.1Media not supportedPermanent: retrying will not help
554 5.6.1 Message content rejected: media type not supported
What it means
RFC 3463 defines X.6.1 as “media not supported”: the message contains a content type that the destination does not handle. Originally that meant a MIME type the recipient’s system could not represent. Today it mostly comes from content gateways that refuse certain attachment types outright.
Where 5.6.0 is vague, 5.6.1 is specific to the kind of content. Executable files, script attachments, encrypted archives the scanner cannot open, and some calendar or voice formats are the usual targets. The decision belongs to the recipient’s policy, not to any internet-wide standard.
A temporary 4.6.1 is unusual, since a type that is unsupported now will be unsupported in an hour. Treat 5.6.1 as a request to change what you send.
Likely causes
- An attachment with a blocked extension such as .exe, .js, .vbs or .iso, even inside a zip file.
- A password-protected archive that the recipient’s scanner cannot inspect and therefore refuses.
- A Content-Type the receiving system has never heard of, produced by an unusual client or a typo in generated MIME.
- A destination that only accepts plain text, such as some ticketing or SMS-gateway addresses.
How to fix it
- Check which part of the message the reply text names; many gateways list the offending file or type.
- Remove or replace the attachment. Share executables and large archives through a file-sharing link rather than email.
- Make sure generated attachments carry correct types, for example `application/pdf` for a PDF, not `application/octet-stream` with a misleading name.
- If the content is legitimate and needed, ask the recipient’s admin for an exception for your domain or for that file type.
Checks that apply
- Email Header Analyzer Shows the MIME structure and content types of a sample message, so you can confirm what the receiver actually saw.
- MX Lookup Reveals whether the recipient sits behind a filtering service such as Mimecast or Proofpoint, whose attachment rules differ from a plain mail server.
FAQ
- Will zipping the file help?
- Often not. Most gateways look inside archives and block the same types there, and encrypted zips are frequently refused because they cannot be scanned.
- Who decides which types are blocked?
- The recipient’s organisation or its mail-filtering provider. Only they can add an exception.