Before you start with this, make sure you completely closed your Outlook Express (not just minimized, you have to shut it down)...
Go to START / Programs / avast! antivirus and choose your MAIL PROTECTION WIZARD.
When it opens, mark (choose) AUTOMATICALLY REMOVE PROTECTION FROM ALL MY ACCOUNTS. Confirm all following requests, and when MAIL PROTECTION WIZARD closes itself, open your Outlook Express. This will remove avast! protection from your Outlook Express, and set original smtp and po3 server names as well as your original logging (user) name...
See if you can send/receive e-mails without avast! mail protection running (no matter if you tried it before, just do it again).
Now, close your Outlook Express again (completely, like last time). Open your avast! MAIL PROTECTION WIZARD one more time, except now you have to choose AUTOMATICALLY PROTECT ALL MY ACCOUNTS, click on NEXT and confirm all requests you get from mail protection wizard.
When you exit the wizard, open your Outlook Express and try to send/receive with and/or without attachments. Note that this procedure will set your smtp and po3 server names to 127.0.0.1 and slightly change your logging name. Of course that's normal behaviour, because avast! must be "sitting" in between your mail client and mail server, otherwise your e-mails wouldn't be protected...
Post back what's happening...
Cheers !