Cannot send attachments any more using the ADSL modem.

I haven't changed my ADSL subscription terms but now I cannot send any
attachments larger than - say - 20-30K using the ADSL modem. There is no
problem using my Dial-up 56K modem which continues to allow me to send large
files, larger than 2-3Mb. Tec. support claims that the problem lies with my
system and even that O.E. is defective for large attachments. This is
patently nonsense because O.E. works perfectly with the dial-up modem.
Besides, even Yahoo Mail now stalls on attachments and it never does with
the dial-up.
This is what happens. Almost immediately after the "Send/Receive", progress
indicator stalls, then O.E. returns the message that the server unexpectedly
disconnected or that it didn't respond in a timely fashion. In Yahoo Mail it
is similar, no message just stalls completely.
I can continue to download large files with I.E. or Navigator. The problem
is uploading using a mail client.

Does anyone have any ideas how to fix this?
Thanks,

Peter