sendmail, queue intervals and ETRN

[David Allen]

I've had the following setup at home for a number of years:

   mailhub <----> smarthost <---> intarwebs

Everything works as intended, of course, but I'd like to change the setup
to accomodate the mailhub host being powered off at night.  After it's
restarted in the morning (or whenever), the mailhub should issue an ETRN
command for any queued mail, and any changes made to the smarthost's
configuration the night before are undone.

Is there a simple answer to this?  What I'm after is a solution that
avoids MAILER DAEMON messages, etc. as a result of the mailhub host being
unavailable.

Thanks.