What you need for email connectivity is determined by:

    1 What networks you intend to connect to.
      The Internet (hence SMTP)?  UUCP sites?  X.400?
      Bitnet?  Others?  Combinations?
    2 What links you have or are willing to install
      Internet T1?  T2?  UUCP?  Other?  [Details on how to
      make your connections is beyond the scope of this FAQ,
      but can usually be found out from the provider (other end)
      of the link]
    3 what user interface you want to use.  This is largely
      an independent issue, so consult the Specific Package
      Reviews directly.