That is configured in /etc/sendmail.cf. There is a good FAQ at
http://www.sendmail.org/faq/
your question may be answered by
http://www.sendmail.org/m4/masquerading.html
I find that if I don't change the sendmail.cf file to masquerade the name, the "domain.com" part is taken from the first "search" entry in /etc/resolv.conf.
Hope it helps,
mlp