[Exim] Help - Controlling bandwidth used by exim

We have an ISP that has 3 remotes POPs in 3 other towns all linking back to
the main server, all the remote pops having been using the main server via
128 and 64 k VPN Frame Relay links for email and auth. Now that they are
expanding this is putting too much load on the links and casuing massive
congestion and dropouts / timeouts.

What I have done is install mail servers at each remote end, because all the
pops use the same @domain for email, i have set exim at the main server to
do a learch of 3 text files, if a user is in a list it forwards mail onto
the appropriate mail server, this has made pop retrieval much better but the
problem now is that if someone is recieving an attachment of say 5mb, when
forwarding the mail onto the remote POP it lags the link fully for the time
needed to send over a 64 - 128 link. My question is, can i set exim at the
main end to only use say 3kps to send email to the remote pops only?? Any
help appreciated.

Thanks
Nathan