How many mail clients are you talking about?  Why not just have them change
their mail server values to something like smtp.domainname.net and update
the DNS forwarder override as such?  I thought that maybe you had something
misconfigured, but tried adding a multilevel hostname and saw that monowall
won't allow it.  So my guess is that might be easier to drop a level on the
client side.  Of course, if you are talking about a couple of hundred
clients or so that is no good.