Outgoing mail doesn't close SMTP connection.
In Mailman 2.1 and Mailman 3 there is a setting (SMTP_MAX_SESSIONS_PER_CONNECTION in MM 2.1, config.mta.max_sessions_per_connection in MM 3) This defaults to 0 meaning no limit.
Whether or not there is a limit, Mailman 2.1's process will call
quit on the connection after delivery to all the recipients of the message has been attempted.
Mailman 3 doesn't do this.
The reason this is an issue is with the postfix MTA at least there may be a milter involved in the Postfix processing, and that milter may have a
close callback which won't be called until the sender sends
QUIT or the connection times out. This can cause delays or possibly other issues in Postfix processing of the message.