[Exim] Exim and dial-up connection

Hi,

I have a Linux machine acting as a server for a couple of other
(Windoze) computers. I'm running into the following problem and
I wonder if somebody could help me or point me in the right
direction: When there are some large e-mails in the exim
queue (usually >600kB) and "exim -qf" (for example) is run
to send them usually the connection is lost completely. When
I start a ping to some computer on the internet, it stops after
some period of time. The internet connection works fine, until
exim is busy sending this large mail. After exim stops (or is
killed) the connection still doesn't work. The ISDN adapter
is still calling, but no data can be transferred. When, instead,
the e-mail is sent directly from the client computer
to the mail server that the Linux server connects to, there is no
problem (but the idea was to collect all e-mails and send them
in one big batch by using cron).

This problem only occurs in the mentioned case. When I FTP
a big file (up- or download), there is no problem at all.

Thanks in advance,
Herre de Jonge