[Exim] exim queue run

Hi,

I have a question about a special behaviour of exim.
My exim is running on a Debian2.2r3 which connects to the internet via
serial modem using ppp. The pppd is on demand. My problem is that each
time exim commits a queue run, the modem dials out. This is not
cost-effective, so I want to make exim stop doing this, but I couldn't
find anything about this topic in /etc/exim.conf. Help me, please.

Thanks in advance,

Balint