Are the clients allowed to SP, er I mean bulk mail? Handling the bounces can be done in many ways:

• Use procmail. A useful recipie involves delivering only the first 50 lines.
• The MIME tools module can be used for this. Keep in mind that certain email clients (Outlook) create strange entities that are very had to parse.
• Merlyn had a script he uses for mailing lists to remove Outlook's extra attachments. You may want to search for that.

Update: These bounces are being returned to whomever sends the bulk mail I hope. Also, I hope the bounces are being removed from the bulk list. Otherwise this is indeed SPAM.