Well I am not an expert at WP, but here is what I can tell you
I used a plug in called check mail in Wordpress. However that works is how I tested it.
If I put the smtp server in the php.ini as smtp.xxxx.ca, it works locally sending to xxxx.ca domain, but if I send to a gmail account, with the queue on hold on the exchange server, it does not show up.
I am running WP on a Windows 2003 server and exchange 2003 on a Windows 2003 server as well.....