I used these sendmail.mc and access files to cause sendmail to use my ISPs smtp server.
/etc/mail/sendmail.mc
include(`/usr/share/sendmail-cf/m4/cf.m4')dnl
VERSIONID(`setup for linux')dnl
OSTYPE(`linux')dnl
define(`SMART_HOST',`mail.example.com')dnl
define(`RELAY_MAILER_ARGS', `TCP $h 587')dnl
define(`ESMTP_MAILER_ARGS', `TCP $h 587')dnl