Share 'Improving email deliverability when using subdomains'

We recently implemented the Ning Premium service for domain masking (http://my.hudforeclosed.net). Since mail is sent from Ning servers posing as hudforeclosed.net, we are having a little trouble with the delivery of invitations and other Ning-initiated mail (particularly with Gmail). In the past, we have enabled everything from DNS and MX records to Domain Keys and SPF records. Should we be loo…