Many ISP''s will only accept outgoing mail from their own account (excluding web based mail of course).
It's a pain - I have the same problem when I do some work from home, everything's fine until I try emailing from my work mail account - Tiscali (and many others) just don't allow it, although you will probably find you can receive mail OK. The only solution I can think of is if your company network has a back up dial up facility (ours still has, but I'm not sure for how long as all our locations are now connected to the net via a LAN). Create a new dial up account using their settings, and just dial in when you want to send mail.
Hope this helps.