I would recommend not even trying to use DH servers to send mail (this is based on the way things were a few years ago; things may be better now but I wouldn't count on it)
I have an SMF forum on a subdomain (like you) and it uses a gmail account to send out all its mail, and it has worked fine for several years.
In the SMF administration panel, under "Administration Center » Mail » Settings", put:
Mail type = SMTP
SMTP server = ssl://smtp.gmail.com
SMTP port = 465
SMTP username = your gmail login
SMTP password = your password
I'm not aware of any problems or difficulties caused by using a different server to send the mail than the one which serves the forum. If anyone knows of any, please say!
P.S. come to think of it, maybe it's aesthetically important to have the mail sent from the domain name which hosts the forum. You can do that if you have a "google apps" account for the domain. But since google apps is no longer free, there is a serious need for a free or nearly free alternative. This would be a good discussion to have.