The trouble with qmail is that I found different qmail versions for different Debian releases. Ubuntu is based on sid, the "unstable" Debian release, but who knows what is the actual relation between the software for which were different qmail packages written, the current Debian versions, and my Ubuntu? I am reluctant to make tests on my Ubuntu server, as I am currently using it. Maybe later.

From my experience, I believe that I have the highest chances to configure Postfix, the mail server supported by Ubuntu project. Too bad that nobody answers my thread there:
http://ubuntuforums.org/showthread.php?t=1266153

Thanks for your help.

I believe I will not try sendmail very soon again. I am using Ubuntu with Postfix now.

Configuring the mail server under linux is difficult because:

1. mail servers under linux can do a lot, they can service thousands of clients over lan and in other configurations,
2. security certificates are a story of their own, and they need to work well with the mail server.

That is why to learn how to configure a mail server under linux takes weeks rather than days.

However, I managed to make postfix send mail (but I failed to make fetchmail receive mail) with this: https://help.ubuntu.com/community/GmailPostfixFetchmail