SMTP
221 Closing Connection
The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
When You See This Code
After sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
How to Fix
No fix needed — this is the expected response when ending an SMTP session. If you see this unexpectedly, the server may be shutting down or timing out idle connections.