http://qref.sourceforge.net/Debian/reference/reference.en.txt

9.6.1. Mail transport agents (MTAs)
-----------------------------------

     For a full-featured MTA, use `exim' in Woody and use `exim4' in Sarge.
     [1] References:
        * `exim-doc' and `exim-doc-html' packages for `exim'
        * `exim4-doc-info' and `exim4-doc-html' packages for `exim4'
        * http://www.exim.org/
     The only reasonable alternative MTA is `postfix' if you care about
     security.  `sendmail' and `qmail' are available as Debian packages but
     are not recommended.

     If you do not need the relay capability of an MTA as in the case of a
     satellite system such as a laptop PC, you may consider using one of
     these lightweight packages:
        * `ssmtp': needs an SMTP connection and is alias-capable, or
        * `nullmailer': can spool but is not alias-capable.
     At this moment, I find `exim' to be more suitable even for my personal
     workstation machine, which is a laptop PC.

     You may need to remove `exim' for the installation of these
     conflicting packages:

          # dpkg -P --force-depends exim
          # apt-get install nullmailer         # or ssmtp

[1]  Following sections use `exim' in examples.  For Sarge replace this
     with `exim4' as needed.

9.6.1.1. Smarthost
------------------

     If you are running `exim4' or `exim' on a host which is connected
     through the consumer grade services, please make sure to send outgoing
     mail through a smarthost offered by your ISP or some others.  [1]
     There are few good reasons:
        * to ensure SMTP retries since your ISP's smarthost usually have
          more reliably connection.
        * to avoid sending mail directly from a _dynamic IP address_ which
          will likely be blocked by dial-up spam lists.
        * to save your local bandwidth to send mails with multiple
          recipients.

     The only conceivable exceptions are:
        * the emergency cure for your ISP's SMTP service trouble.
        * an experiment for the educational purpose.
        * your host being a professionally hosted server.

[1]  You must follow this rule for any hosts on dial-up, DSL, cable
     services or LAN through some broadband router.  Even if your home host
     has a fixed IP from your ISP, it is still a good idea to follow this
     rule.  Most workstations and home servers fall into this category.