e-mail return paths, mmdf?

[dave davis]

This is probably a trivial question, but without adequate documentation, I
can't seem to find a satisfactory answer.  I'm trying to locate the source
of a problem with an application which is supposed to respond to e-mailed
queries.  The problem we are seeing is that mmdf (I think) can not send a
response back to the users.  I was under (the possibly erroneous) impression
that if a message sent from site A can be received at a site B, then a
response could be sent from B to A using the return path.  Is this the
case?  If so, are there any limitations to the length of the return path?
(i.e., if the application needs to store the return path and be reasonably
sure to cover the 'worst case,' what length path is needed?)  Do some
versions of MMDF handle this better than others?

Sorry if this is a RTFM type question; if you have a source of information
which explains the inner workings, I would be very greatful for the
information.  Thanks.

dave