SMTP code 4xx
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
- Temporary failure: the message was not accepted
- What the sender does
- Put it back in the queue and retry later
From M3AAWG Sender Best Common Practices (Version 4.0) · Bounce and NDR handling
This line comes from a knowledge base article on corporate email gateways, not on the standard.
- Next step
- Slow down, verify the IP address's health, retry; delist if it persists
- Symptom
- 4xx deferrals across many unrelated business domains at once
- Likely meaning
- Dynamic delay in the style of PDR, or throttling of a new IP address
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
This line comes from a knowledge base article on Yahoo, not on the standard.
- Cause
- Yahoo mail servers were busy at the time of connection
- Next step
- Usually brief. Retry after a short period
- Category
- Resources temporarily unavailable