How can Spammers abuse my apache sendmail?

[nhawk]

There are two places for each usergroup where email can be sent.

1) Can Use Email to a Friend
2) Can Email Members

Be sure both of those are turned off for these usergroups..

Unregistered/Not Logged In
Users Awaiting Email Confirmation
Users Awaiting Confirmation

and optionally..
Registered Users
Any other usergroups you want to turn it off for.

If mail is still being sent out after doing that, then it's one of four things..

1) You have an add-on that's sending the mail.
2) Your mail account has been hacked.
3) Your site has been hacked.
4) Your server itself has been hacked.