Finish or continue after a 521 opening
02A 521 opening reply does not impose one mandatory immediate-close behavior. QUIT follows the normal SMTP rules; other commands have two permitted server responses.
Alternative reset after the opening 521: an extra EHLO arrives, and this server chooses the other permitted behavior, closing without an additional reply. It is not a later step required after repeating 521.
Source for this drawing: RFC 7504 · Read RFC source: §3; normal QUIT: RFC 5321 §4.1.1.10 ↗
New to this? Start hereA quick picture + helpful background
A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
Helpful first, if these ideas are unfamiliar
These pictures are analogies to help you learn. The linked RFC explains the actual rules.
Terms in this drawing
Related RFCs and changes
“Updates” changes part of an earlier RFC. “Obsoletes” replaces it. This list shows selected related documents.
After 521 replaces 220, the SMTP session proceeds normally. QUIT asks to end the session; RFC 5321 requires the server to send 221 (MUST) and then close.
- After 521 replaces 220, the SMTP session proceeds normally. QUIT asks to end the session; RFC 5321 requires the server to send 221 (MUST) and then close.
- For additional commands other than QUIT, the server MAY keep returning 521 or MAY simply close the connection. These are alternatives, not successive mandatory steps.
- Closing after a non-QUIT command is often preferable for resource conservation, but that preference does not turn either MAY into MUST.
Fictional example.com and example.org names. Separate comparisons reset the scene; chosen reply text and timings are not protocol defaults. This is an educational model, not an SMTP implementation.
Read the RFC section ↗