PHP/Sendmail interaction : how to get rid of wrong emails

Hi all I'm facing the Problem that lots of my users are too stupid to write their email correct. When i mail them with php a return email is send to me by their mailserver, which results in LOTS of mail for the root account. Can i get sendmail to put the returned emails in a file? so that i can check that file with php and delete those mails from my DB ? or is there a way to check the email when they enter it in a form (online versus their mailserver)? i already check for wrong email syntax, but that does not solve the problem... Sebastian