Problems with outgoing emails in Outlook 2002

My boss is having problems when he trys to reply or forward messages. It is
an intermitten problem. When he hits send, he immediately gets a system
admin email back with a non deliver error 550. Can you tell me how to fix
this. We have looked at settings in his options and don't see anything
helpful. He runs MailWasher Pro 4.0. We tried turning MailWasher off and
problem still exists. HELP! Thank you.