On sending mail the session is ended (by disconnection) before the complete multi-line error message is recieved.
No QUIT command is issued. 

Claws log:
[13:29:13] ESMTP< 235 Authentication succeeded
[13:29:13] ESMTP> MAIL FROM:<eg@example.com> SIZE=420
[13:29:13] SMTP< 250 OK
[13:29:13] SMTP> RCPT TO:<blackhole@example.com>
[13:29:13] SMTP< 250 Accepted
[13:29:13] SMTP> DATA
[13:29:13] SMTP< 451-Quota exceeded.
** error occurred on SMTP session
*** Error occurred while sending the message:
451-Quota exceeded.

Telnet session:
MAIL FROM:<eg@example.com> SIZE=420
250 OK
RCPT TO:<blackhole@example.com>
250 Accepted
data
451-Quota exceeded.
451-This message is (1 x 420) bytes,
451-but only 2 remain.
451 Try again on 6 Mar
quit
221 example.com closing connection

RFC 5321 says:
"An SMTP connection is terminated when the client sends a QUIT command.
The server responds with a positive reply code, after which it closes the connection."

In this case the missed message contains useful information and should certainly be logged
and/or popped up in a dialog. Other servers may be less forthcoming, naturally.

Cheers,

Martin (kMail refugee).