[Zope] Anyone using Zope Mail Host with MS Exchange ?

I have a client reporting that mail sent from Zope using a MS Exchange box as their Zope Mail Host never arrives. The forms themselves are known not to be at fault since we have previously tested them using a Zope Mail Host defined on a unix box (running QMail). I can test the forms from remote as an end user and they seem to submit properly - so the mail host is accepting the messages - but they never arrive anywhere (not even in their postmaster a/c) Since they are doing this from behind a firewall and refuse to open it for ssh access, I thought I'd first check if this is a known issue before I take a flight there to sort it out. Thanks in advance, chas