[Tim Wood]

Just to be clear, the "after" bit means that if your email address is "foo@bar.com", you would use "foo+omnifocus@bar.com"

Now that I look at it, this isn't clear at all from the preferences pane and is only understandable if you know this is part of the standard for email addresses. I'll make a UI bug to add some explanatory text here.

Still, some mail servers are non-conforming, and will reject or strip the "+" notation. This is one reason there are two options for recognizing mail intended for OmniFocus. If the '+omnifocus' doesn't work for you, you could try switching mail providers if this shortcut is important to you (you can add the +omnifocus address to your Address Book and have it complete in your iPhone's To field), or you can switch to the subject prefix approach.