Exact response pattern
221 Service closing transmission channelInfoGenericProtocol
What it means
The SMTP server is closing the connection normally.
Likely causes
- QUIT was issued.
- The server completed the session.
Recommended response
- No corrective action is required unless the close was unexpected.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.