non-dedicated SMTP

We have a gateway machine that people dial into to get SMTP/UUCP mail. If
they are SMTP based, they either have to do an ETRN or usually a
"rsh /usr/lib/sendmail -qRdomain.com" to get their queue flushed.

Since exim uses a database of unreachable sites, it makes the possibility
of a running a loop that checks for connecting sites plausable. Thus, the
client doesn't need an ETRN or an rsh, they just need to connect.

Does anyone have any ideas how this could work?