Is a 5.6.0 a spam rejection?

Not usually. It points at the format of the message. Spam and reputation refusals almost always use 5.7.x codes instead.

Why do some recipients accept the same message?

Parsers differ in strictness. One server tolerates a broken boundary while another refuses it, so the fix is to make the message valid rather than to rely on lenient receivers.

5.6.0 bounce: other or undefined media error

5.6.0Other or undefined media errorPermanent: retrying will not help

Typical wording

554 5.6.0 Message rejected: invalid content

What it means

The X.6 subject in RFC 3463 covers “message content or media status”: something about the message body, its MIME structure or its headers got in the way. Detail 0 is the unspecified member of that family, used when a server dislikes the content but has no narrower code for it.

Real-world 5.6.0 replies often concern malformed messages: duplicate or broken headers, MIME boundaries that never close, or line lengths far beyond the 998-character limit of RFC 5322. Some gateways also use 5.6.0 for attachment policies, although those more often carry 5.7.x.

A 4.6.0 reply means the receiver had a temporary problem processing the content, for example a content scanner timing out. Your server will queue and retry that one. The permanent 5.6.0 needs a change to the message or its generator.

Likely causes

An application that builds MIME by hand and produces unterminated multipart boundaries or wrong Content-Type headers.
Header lines that exceed 998 characters or contain raw 8-bit bytes without an encoding.
Duplicate single-instance headers such as two From, Date or Subject lines.
An attachment type or archive the recipient’s content filter could not open and chose to refuse.

How to fix it

Open the bounced message source and look at the headers and MIME structure, not just the visible body.
Run the headers through a header analyzer and correct duplicate or overlong fields at the point where the message is generated.
Let a mail library such as Python’s email package or Nodemailer build the MIME tree instead of string templates, and encode non-ASCII text with base64 or quoted-printable.
If an attachment is involved, send a version without it; if that delivers, share the file through a link instead.
Still stuck? Ask the recipient’s admin which content rule fired. Only their filter log knows.

Checks that apply

Your domain or sending IP (fills in the checks below)

Email Header Analyzer Parses the header block of a sample message so you can spot duplicate, malformed or overlong fields before you resend.
MailFlow Monitor Sends a test message through a monitored path and confirms that a corrected template now arrives intact.

FAQ

Is a 5.6.0 a spam rejection?: Not usually. It points at the format of the message. Spam and reputation refusals almost always use 5.7.x codes instead.
Why do some recipients accept the same message?: Parsers differ in strictness. One server tolerates a broken boundary while another refuses it, so the fix is to make the message valid rather than to rely on lenient receivers.

All 191 bounce codes