[exim] limiting simaltenous connections to certain hosts

Hello,

There is a provider here in Poland offering free accounts and they set
up an (I think)
unreasonably low limit of simaltenous connections (about 3 if I am not
wrong) from a single IP. Because I have about 1300 subscribers with such
domain, the delivery is made difficult and takes considerable part of
the day.

Is there a way to have a router which would limit the number of sim.
connections to
hosts defined in a flat file so that exim does not unnecessarily keep
trying where most of its attempts are being rejected anyway?

Many thanks for suggestions!

Warm regards,

Zbyszek