How to add DKIM feature to the IIS 7.5 - Windows Server 2008
DomainKeys Identified Mail
There is virtually no way to do this within IIS 7.5 (or any version of IIS for that matter). There may be a way to hack it if you write your own module, however that's not so much an IIS hack as it is your own hack that plugs into IIS.
There are third party tools that exist to perform this. For example, EA DomainKeys/DKIM for IIS SMTP Service and Exchange Server from AdminSystem Software. However, for that price, I would suggest that you consider using a real MTA that will serve you much better in the short and long terms.
Consider some of the following products which are legit MTAs that will not need to be hacked or kludged to perform the functions of a real mail server: