doesn’t have to enter them?

Yes.  The POP3 server used is referred to in the HTML
templates by the substitution keyword “SERVER”.  Hunt down
everywhere that MailMan allows the user to supply this value
through a form field.  The only place where this should
happen is in the template “t_login.htm”.  Find this
form field:

         <input type="text" name="SERVER" size="30">

And replace it with:

         <input type="text" name="SERVER" size="30"
               value=”popserver.mydomain.com”>

The box will now be filled in by default for the user when
they log on, but experienced users will still be able to aim
MailMan at specific servers.

The SMTP server used by MailMan is specified by the keyword
“OUTGOING”.  Change the form fields that set the “OUTGOING”
keyword in both “t_f_messageform.htm” and
“t_nf_messageform.htm” in order to rig the outgoing SMTP
server to something specific.