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

[chas]

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