SMTP code 5xx
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
The knowledge base documents this code in more than one row. What tells those rows apart is the rejection text in your own log, not the code: read each line below against what your rejection says.
- Meaning
- Permanent failure: the message was not accepted
- What the sender does
- Do not put it back in the queue
From M3AAWG Sender Best Common Practices (Version 4.0) · Bounce and NDR handling
- Test that tells causes apart
- Run a seed test or a spam check on the exact message, and remove elements one at a time
- Next step
- Fix the content (content and design): remove URL shorteners and fix the HTML
- Rejection says…
- Mentions content, links or attachments, "message content rejected", or "spam" in a 5xx
- Hypothesis
- A content filter blocked the message
From Delivery Troubleshooting Playbooks · Playbook A: message not delivered (bounced or rejected, 5xx)
This line comes from a knowledge base article on corporate email gateways, not on the standard.
- Next step
- Delist through the vendor portal; fix the root cause first
- Symptom
- 5xx at connection or early in the SMTP session, citing reputation, a DNSBL, or a reputation URL
- Likely meaning
- Global reputation block (PDR, CSI, BRBL or Spamhaus)
From B2B / Corporate-Gateway Deliverability · Interpreting gateway bounces
This line comes from a knowledge base article on corporate email gateways, not on the standard.
- Next step
- Only the recipient organization can fix it; ask your contact to request allowlisting
- Symptom
- 5xx citing "policy", "local policy", "prohibited by administrator", or a rule name
- Likely meaning
- Tenant administrator's policy
From B2B / Corporate-Gateway Deliverability · Interpreting gateway bounces
This line comes from a knowledge base article on Yahoo, not on the standard.
- Cause
- Duplicate headers, badly formatted headers, incorrect MIME types, and other violations of RFC 5321 and 5322
- Next step
- Fix how messages are built so that they comply with the RFCs
- Category
- RFC compliance failures