I have a Delorme Inreach Explorer.. It is a fantastic device and has very affordable plans (can be "parked" when not in use).
It doesn't technically send e-mail, more like text messages. You can send a 160 character text message to any cell phone
or e-mail address and they can reply back.
It can also post 160 character messages to Facebook or Twitter. Unfortunately it does not show the comments on the posts though, but it does allow a reader to send you a message or track you on a map.
One thing to note.. It does not work indoors or close to tall buildings. It needs a clear view of the sky to function well (a boat roof is no problem).
Here is a link:
Look on Craig's list, you can sometimes find them used.