Sending mail > 15kb won't work

Hi,

I have been experiencing a problem that I have not been able to fix up until now. I wonder if any of you have any suggestions, as I am quite stuck.

I had an initial setup of two PC's: one with Windows XP Home Edition and an internet connection through ADSL. Connected through a LAN was another PC, with Windows 98 SE. That connected to the internet through menas of the first PC, that had Winroute Lite installed to share the ADSL connection. I need Winroute because I have not yet found a way to share the ADSL connection through WinXP only, because it is set up as a VPN-connection.

Anyway, everything worked fine until two weeks ago, when the Win98 machine suddenly refused to send e-mails larger than roughly 15kb. If I try and send a mail of say 45kb, the progress bar stops dead at 33% until the connection to the mailserver times out. This occurs in Outlook, Outlook Express, Mozilla and even webmail (!).

As this PC was scheduled to be replaced, this seemed like a nice moment to do so, because I suspected Win98 configuration problems and didn't want to go through the fuss of reinstalling. So I replaced the Win98 machine with a brand new and completely clean PC, also with WinXP Home. I was surprised to find that the exact same problem reoccured.

So I figured the problem was with the gateway PC, so I cleaned it up, reinstalled WinXP, also installed SP1 on both machines, but STILL the problem occured. Every other internet function worked fine, except for the mail problem, although the PC functioning as gateway has no problems with e-mail at all...

So now I'm stuck... I tried reinstalled Winroute, but no success. Some people have suggested it could be MTU problems, but I'm not familiar enough with networking to work out if that is true or not and how to fix it. I am really baffled by the fact that the very same problem persists after two completely clean installs. Btw, I tried connecting a third PC (my laptop, also with WinXP) and it shows the same e-mailing troubles.

Can anyone figure out what to do? I'm kinda desperate...

Thanks a bunch!