What is “spam content rejected”?

Spam content rejected indicates content-based filtering:

What triggered rejection:

Spam trigger words or phrases. Suspicious URL patterns. Excessive punctuation or formatting. Image-to-text ratio issues. Known spam signatures.

Filter mechanisms:

Pattern matching against known spam. Machine learning classification. Reputation of embedded URLs. Attachment type analysis.

Typical codes:

550 or 554 with "spam" or "content rejected." 5.7.1 (message rejected due to content).

Response:

Review message content for spam triggers. Avoid spam-associated words and patterns. Check URLs for blacklist status. Improve authentication (SPF, DKIM, DMARC). Consider deliverability testing before large sends.

Spam content rejection is the filter saying "this smells wrong." Clean up your message content.