Currently kmail on my system will keep a TCP connection to the mail server
open after a message is sent, it stays open until the mail server times out
and disconnects it.

I believe that kmail should send a "quit" command to end the session.  Keeping
the connection open serves no purpose (no great effort or time is taken to
establish it) and acts as a minor DoS attack on the server.  For example most
of the mail servers I run have a maxproc limit for the SMTP service that is
significantly smaller than the number of users.  If all users on such a
network ran kmail and most of them tried to send email within the 5 minute
timeout period (which happens at about 5PM in some of the offices where I do
sysadmin work) then some users would be unable to connect.