[Exim] bidirectional relaying

Hello,

I have a small, remote, business network that has a pay-per-minute dialup
connection to an ISP.
Everyone shares this link for internet access (the gateway/firewall is a
linux box using diald).
I want to set up an external mail relay for these users. Outgoing mail
seems simple enough; however, incoming mail is tricky -- as the firewall's
IP address is dynamic and no domain name exists for it.

It would be great if this firewall could do a client connection to the
machine relaying in order to
initiate the relaying of mail (rather than having the relay try to push the
mail nowhere in particular).

Is this possible? Can I do it with exim? Any feedback would be excellent.

Regards,
Andrew Bell