Big mail problem

First of all, thanks for reading (in advance). As stated this one is a real
mystery. Up front synopsis . . . . . regular e-mail can reach the intended
recipient but not PHP form results. Now the details -

First issue was not being able to receive mail via Outlook. Browser based
mail fine - but not Outlook. So we decide to host the mail in-house rather
than at the domain name. We change the A record to send mail to the IP
address of the in house server and completely turn off all mail coming to
the web server. Now Outlook works. However the forms on the website will not
send the results to the same e-mail accounts on the in house server that are
receiving mail just fine.. These are forms built using Flash so we convert
them all to PHP instead. They still don't reach the recipients. The forms
will send the results to anyone and everyone listed in the recipients
(including a Hotmail account) but not to any e-mail address on the in-house
server that is hosting the mail.

So the question is . . . . . is the in house server rejecting the forms
results and if so why? Or is there another issue? (doubtful) BTW this is a
Linux server. Help! I need a hero!