I have configured Sendmail to work with a smarthost by adding the following line to the configuration
define('SMART_HOST', 'smtp.ISP.TLD')dnl
and after that I issued
make -C /etc/mail
The mail log shows
Aug 8 17:51:23 x3tech sendmail[10677]: o78FpM8q010677: from=XXXXXXXX, size=64, class=0, nrcpts=1, msgid=<201008081551.o78FpM8q010677@mainframe.domain.tld>, relay=root@localhost
Aug 8 17:51:23 x3tech sm-mta[10678]: o78FpNmK010678: from=<XXXXXXXX@mainframe.domain.tld>, size=360, class=0, nrcpts=1, msgid=<201008081551.o78FpM8q010677@mainframe.domain.tld>, proto=ESMTP, daemon=MTA-v4, relay=mainframe.domain.tld [127.0.0.1]
Aug 8 17:51:23 x3tech sendmail[10677]: o78FpM8q010677: to=XXXXXXXX@gmail.com, ctladdr=XXXXXXXX (1000/1000), delay=00:00:01, xdelay=00:00:00, mailer=relay, pri=30064, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (o78FpNmK010678 Message accepted for delivery)
So it seems it never passes on to the smarthost. Oh yes, and I'm using Debian 5.0