It is very easy to setup sendmail so that it will pass your email off another smtp server instead of just sending it directly.  This would be required when your ISP doesn’t allow you to sendmail without passing it through their servers.  You would need to edit /etc/mail/sendmail.mc and then just does a restart of sendmail:

define(`SMART_HOST’,`smtp.myisp.net’)
MASQUERADE_AS(`dbaron.org’)
FEATURE(masquerade_envelope)

Make sure you remove the “dnl” from the beginning of the line as that is the way sendmail comments out the lines.