Email With Attachments Won't Send


I work on a G4 Quicksilver PowerMac, OSX 10.3.9, 1.25GB of RAM. Since my latest upgrade to Panther, I have experienced sporadic problems in Mail when sending graphic files to my clients as attachments. Simple messages send without a problem. I can also receive without a problem. Recently the problem is constant; if I attach anything to a message it tries to send, but eventually I get a message saying it can't send it and the message will stay in my out box until it can be sent. "Sending the message content to the server failed."
Along with this problem has come another annoying one (little stamp jumping up and down); I am constantly being asked for my password: "POP server rejected the password for user (me). Reenter password or hit cancel." I reenter and it reconnects with the server (Broadband wireless)or I just quit mail and open to check it periodically.
Doing online research, I found several people with the same or similar problem and the one alternative I tried that worked (I haven't trashed anything or re-installed yet) is opening my OS 9 and using Outlook Express. I can send and receive fine from there. I suppose I could just use Outlook, but it is more convenient to use Mail. BUT I'm flexible.


BTW -- this is a terrifc website.