Can't send larger files from .Mac desktop

Hi everybody,
I can't send larger e-mails from my .Mac desktop. Particularly mails with attachments above 20Kb, i.e. Excel, Word, PDF files.
The error message is:
'This message could not be delivered and will remain in your outbox until it can be delivered. Sending this message content to the server failed.'

Smaller e-mails with the same software attachments go through.
I have already sorted out the following issues:
1. my ISP allows me to send e-mails with up to 10Mb
2. I can send the large files from my .Mac webmail account and other webmail accounts like hotmail to my .Mac desktop and open the attachments.
3. I can not send the same e-mails back.
4. My outgoing mail server is smtp.mac.com, server port 25, SSL. I understand I could use the outgoing server of my broadband provider (Telewest UK) but since I do not like their service, I'd rather not
5. I have checked my .Mac preferences and they look normal

I have a iMacG5 2GHZ, use Tiger OSX 10.4.2., use .Mac mail 2.0.2, have a Netgear wirless router. Am a new Mac user since June and have paid for the .Mac package £40, so it is my first disappointing experience with Mac. So far I have been in seventh heaven with the whole set up.

Maybe
- I have set up .Mac incorrectly
- it is a firewall problem
- a router problem.

Has anybody got an idea?