http://www.priacta.com/troblog/2008/06/30/training/sending-emails-as-tasks/
How to Do It When It Is Appropriate
If you decide it’s appropriate to send tasks (or send emails as tasks), here’s how to do it.
1) With Trog Bar. If both you and your recipient have Trog Bar software, just copy the email into an “Actionable Email” folder in their Outlook folders and Trog handles the rest. From there they can process it and/or click Open and hit Reply as a regular email. First create a folder (usually called “[Action]“) in their Outlook folders. Then tell Trog about this folder by clicking the [+] options button, selecting Profile and Folder Options, and assigning the “Actionable Email” category to it. Then set things up you you can put emails into their folder using one of these methods:
a) Exchange Server or shared Outlook folders. If you share folders over Exchange Server or some other way, you can assign tasks to them by dragging and dropping emails into their shared folder. Trog displays them in the Unprocessed Tasks list automatically. With Exchange Server, share the folder per the detailed instructions here.
b) And/Or Outlook Rules. For a lower-tech approach, hit Reply and add a “(W/F:Sue)” tag to the end of the Subject line. This says “I’m delegating this to Sue, and I’m Waiting For a reply.” Beforehand, set up a rule in Sue’s copy of Outlook to move all INCOMING emails with “(W/F:Sue)” in the subject into her [Action] folder. On your side, set up a rule to move a copy of all OUTGOING emails with “(W/F” in the subject into your own [Action] folder. This automatically puts a “Waiting For” task in your Unprocessed Tasks list if you delegate via email to anyone. Way cool.
2) With Outlook alone. You can follow the steps for #1 above in Outlook without the Trog Bar. Instead of dragging the email into a shared [Action] folder, drag it into a shared task list. The created task wil have no categories, which means it still needs to be processed.
3) With other tools. Remember the Milk supports Total, Relaxed Organization (TRO) and lets you send tasks into another person’s Inbox. See here for instructions on sending tasks to your RTM contacts. A version of the TRO Field Guide for Remember the Milk is due out shortly, with detailed self-training and coach-assisted training instructions, so stay tuned. Some other tools have similar features, especially if they support autoprocessing of emails into tasks. See the GTD Software Comparison Table and search for the word “autoprocess” on the page (hit Ctrl+F to search) to find those apps.