Need Option to Send Email to Existing Users

We have a need to be able to send email to existing users. Actually, we're using this so that we can easily see what the letter will look like that we send out without having to delete our user's email account each time. But I imagine there may be some other uses for this.

Anyhow, I've got a version working on 6.x-2.3, but I imagine it would be better to make a patch against 6.x-2.x. So, let me test what I've done for a bit and confirm that it works ok. The option to send email to existing users is set inside of the "Update Existing Users" box.

I also started working on some code to offer a button that would send an example letter to the email address of the account holder who is doing the import, but that seemed trickier to do.