Introduction to DomainKeys/DKIM for IIS SMTP Service and Exchange Server
With this tool, your IIS SMTP Service and Exchange Server will be enabled to add DomainKeys/DKIM signature to outgoing emails from specified domain(s), which verify the DNS domain of an email sender and the message integrity, and in turn preventing the emails from your domain from being flagged as spam or junk mail by email servers which have Domainkeys/DKIM implemented (such as Hotmail, Yahoo mail and Gmail).
See Also
Using DomainKeys/DKIM Sink
Deploy public key in DNS server
Test DomainKeys/DKIM signature
Appendix - Set up SPF record in DNS server