DomainKeys/DKIM is a popular method for email authentication. It is recommended to setup DomainKeys/DKIM record for your domain.
Please refer to EA DomainKeys/DKIM for IIS SMTP Service and Exchange Server
http://www.emailarchitect.net/domainkeys/default.asp
If you have any problem in DomainKeys/DKIM/SPF record implementation, please contact support@emailarchitect.net.