First up, if you aren't running a mail server you can simply turn sendmail off and stop it from starting.
Check /etc/hosts and see if there is a line like
127.0.0.1 localhost localhost.localdomain <the_full_name_I_gave_to_this_computer_what_ever_it_maybeincluding_any_dot_com_bit>
or similar, basically is their an entry in /etc/hosts that mentions anywhere the name that you have given your computer ?
Sendmail is probably trying to resolve the hostname