Rinja,
This would depend on how you have configured sendmail to send outgoing e-mail.
all outgoing mail is stored in a directory on your server before it gets to the outside world. If you have configured sendmail to 'delay' sending e-mail then you could look at it beforehand.
if you want to 'hive' the e-mail to a seperate directory then that would be quite simply to do. If you wanted it to be e-mailed to yourself then thats harder and I haven't managed it myself, but there is a posting on the sendmail newsgroup that describes what to do.
there is also a comercial product 'sendmail switch' that can do it.
hope that helps
Mike