Hi John,

It is my understanding that sendmail is the most capable/functional mail system under unix/linux. Qmail would be a kind of light version of sendmail and is probably easier to setup and administer. This is just my guess because I've never really used qmail.

This is from "Running Linux", 2nd Edition:

The smail Mail Transport Agent
--------------------------------

There are two major MTSs available on Linux: smail and sendmail.  They both
support the SMTP and UUCP protocols for handling mail.  The sendmail package
has been around for a long time.  It is generally considered somewwhat more
difficult to use than smail, but it is thoroughly documented in the book
sendmail, by Costales, Allman, and Rickert.  The smail package is reportedly
eaier to configure and manage than sendmail, and is perhaps more robust on
systems with limited memory.  It is very popular on Linux.  We've decided to
go with the most popular Linux MTA, smail, and describe its essential
configuration and use here.  However, if you need to configure a mail host
that has many connections and handles lots of mail, you really need sendmail.

This may be somewhat out of date as Running Linux, 2nd Edition came out in June of 1996, and the current verion is 3rd Edition.

I checked Oreilly.com and the current version of the book sendmail is:

sendmail, 2nd Edition 

By Bryan Costales
With Eric Allman
2nd Edition January 1997  
1-56592-222-0, Order Number: 2220
1046 pages, $49.95 US $72.95 CA £35.50 UK