Thanks to the Yellow Rose of Texas for pointing this out to me. (Yes, it's no secret. I just hadn't noticed. I'd noticed the SMTP etc. business in the headers and junk mail errors that looked a _whole_ lot like regular mail, but...)
cMail isn't an internal message system like private messaging was. ct96574 @ cherrytap.com (or eschiss1- to use the examples of my own addresses) apparently is now an e-mail address that can receive mail from and send mail to within and without the site.
Well, neat.
