How to reject ftp users from sending mail

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http://www.linuxquestions.org – Hello there, Is there a way to reject ProFTP users from sending mail through sendmail? All ftp users have /bin/ftponly set as their shell and are all in seperate groups. There are thousands of ftp users on the server. There are also thousands of mail users which have /bin/mailonly set as their shell and are also in seperate groups. Can users with a certain shell being blocked within the sendmail configuration? Or is it an option to use iptables for this? (HowTos)