I dont know much about mac's.

I have a user that has been using her mac with outlook express Ver 5.01 for the last year or more without issues. seemingly out of the blue (week ago), attachments will no longer work. you attach a file and try to send and it times out with error (-3155 could not send mail to "my account"). I can recieve mail, and i can send mail without attachments.
I talked to the ISP, they say everything is fine on their end, i believe them because i can add a different mail account to the mac and still get the same problem. i disabled the virus scan at the suggestion of the ISP, no help. I also checked, double checked and messed with my mail settings, they are back to normal now.
I installed Eudora after much distress, on installation i imported account settings/messages from OE, and Eudora has the exact same problem. i can send mail, just not attachments, they time out complaining about a network problem.

Does anyone have any ideas on what is happening here and how to fix it.
thanks.

Try sending a txt document with one letter in it.

If it works then the network won't allow attachments to be sent if they are over a certain filesize. If it still doesn't work, you're boned.