There's no way to do this through the RUI nor the command line tools. You'll have to go through an API client.
If you're sending exactly the same email to everyone, this is pretty easy to do. The MTurkR package for R has a graphical interface that includes the ability to send messages in bulk (see here).
If the messages need to be different for every worker, things are a bit more complicated. Again, in MTurkR, this is pretty easy but you have to be a bit familiar with with the R language for it to be perfectly intuitive. In short you can use the ContactWorkers function to send individualized emails. You simply need to get the workers WorkerId numbers into R, then you can modify a template text using simple text manipulation tools in R. The documentation for the package shows how to use the function.
It is, of course, also possible to do this in a variety of other languages (Python, Ruby, etc.), but R is the language I work in and it's typically reasonably easy to use for those with no programming experience to use. (And for full disclosure, I'm the author of MTurkR.)