As a system administrator, I have experienced sendmail, qmail and postfix for mail servers.  Even after I started to use Debian, I continued to use postfix.   But default MTA for Debian is exim4.  And now that I am no loger a mail server's administrator,  I feel it's time to try exim4.   For several months, I didn't know how to understand exim4 configuration.  But finally found hint in /usr/share/doc/exim4/README.Debian.gz     apropos exim   Well, I didn't imagine Debian specific configuration guides are in man pages.   man exim4-config_files   Still, I don't know exim4 configuration syntax, but I can setup a smart host for my laptop PC.