I just implemented a cloud service using SendGrid , which accepts emails at [workitemid] @ mycloudservice.com and adds an email to the work item history. Therefore, if you send an email to the address 1234@mycouldservice.com , the date / subject / body of the email is added to the item ID 1236 "Work History".
Now I have a lot of things hardcoded for my personal use, but I'm going to make it universal / customizable for public use. The big caveat is that the TFS server must be visible on the Internet (works great with Team Foundation Service). I will update this answer when (if) I open it for general use. The code that SendGrid POST handles is here if you're interested.
There is also a TeamCompanion Outlook add-in, which gives you a button in the ribbon to attach the current letter to the work item. In the last round, I went with TeamCompanion, I found it in order, but a bit wrong, and this may have slowed Outlook (or it could be psychological).
source share