Look like certain websites possible hacked. Investigate 'maillog' in 'admin > log manager'.
For awhile, try disable sendmail with add '/home' in 'admin > sendmail to bans'. With '/home' added, it's make all sendmail will be failed.
If you sure, sendmail from certain website, you can change '/home' to '/home/<user>/<domain>' (aka document root of certain website). Or change to '/home/<user>' (it's make sendmail from all websites under this user will be banned).
After investigate you may to know where sendmail come from.