The standard edition of MailMan is more of a 'viewer' for a
POP3 server than a complete mailer.  It does not store any
user information on the server side.  It does not store
messages anywhere, so the only messages that you see are the
messages that are actually on the POP3 server at that time.
The professional edition stores messages on the server side
after they have been fetched from the POP3 server, allowing
you to organize them into a folder hierarchy like you might
in Eudora or whatever mail program you ordinarily use.  The
professional edition also stores user settings on the server
side, allowing users to configure options such as a default
SMTP server or a signature message to append to outgoing
messages.  The standard edition is a version that we intend
for use by users as a secondary mail system that will
compliment an existing client-side mail package, the
professional edition is intended for use by novice users as
a primary mailer.  Experienced users will probably not be
happy with any version of MailMan as a primary mailer, just
like they would not use HotMail or ExciteMail for a primary
mailer.

Another difference, on a more practical level, is that the
professional edition is not yet complete, and won't even go
beta for at least two or three weeks.  (This was written on
Wednesday, May 27, 1998, during the public beta of MailMan
version 2.0 Standard)