Mail attachment limits.

[#1]

Hi,
i'm unable to send from home an e-mail with even a relatively small attachment (200k).  i can upload attachments to the web mail site and send them that way without trouble.
My client appears to transfer part of the file (goes to a fixed percentage done) and then claims the SMTP server failed (see attached dialogue).
i'm using Thunderbird, which imposes no limit.  WinXP Pro SP2.  No outgoing mail scanning.
My problem is the same one reported in the archive, http://community.plus.net/forum/index.php/topic,28044.msg254444.html#msg254444.  That was unresolved.
Anyone have any ideas?
Cheers.

[#2]

Firewall and antivirus apps would be my first port of call. Try temporarily disabling these and see if there's any difference.

[#3]

Hi Bob,
Thanks for the suggestion, but it makes no difference (unless i've carelessly missed one).
tip.

[#4]

This has got to be something to do with Plusnet, because i can use the same machine and mail client to send these mails via gmail and hotmail.
Doesn't anyone want to make me look really stupid by suggesting the one obvious thing i didn't think of?    There must be more than two people in the world who've ever had this problem.
                          tip.