The below screencast is one of nine that I did for the new version of TextExpander for the Mac. At the end, I added a bit about automating email subject lines and message bodies with one snippet and I seem to have touched a nerve. I've received a ton of email about this. A lot of people didn't realize this is possible.
The trick is the Tab key. In most mail applications, the tab key moves you from the subject line to the message body. Setting the curser in the subject line, you can have a TextExpander snippet type the subject line, the tab key, then the message body. For instance:
Subject: How about some waffles?
I'm really hungry for some waffles. How about you?
If you are automating email that includes a standard subject line this can be a huge time saver. Don't forget you can also include variables and fill-in snippets in the subject line as well. For instance, if you are standardizing the email for your monthly invoice, the subject line snippet could be:
ACME %b Invoice
which would render as:
ACME June Invoice
Anyway, if this stuff interests you, it wouldn't hurt to watch the below screencast. If you'd like to see more in this screencast series, click on the "Playlist" button. I'm proud of all of them.