Should I decide to print some letters sent our way, and someone provides no other way to refer to them (ie, their “name” is their email address), is it bad karma to post that address? I’m guessing yes, but if I post “r33t4rd598 sent this email”, I’m pretty sure people will start to figure out @hotmail@yahoo@whatever pretty quick?
Instantly delete any e-mail by people who are too rude or too stupid to provide an actual name. Problem solved. I bet you won’t lose any worthwhile letters that way.
If they didn’t provide a name at the end of the email, then I see no problem with just going by the email username, sans the service.
I would suggest putting a notice near where you have your contact email listed suggesting they submit a name and/or location within their emails, just for the hell of it.
Doesn’t help you at the moment or at all if you don’t have control over such things, but it might be worth setting up a web form for people to use for feedback instead of accepting e-mails at all. You can make “name” required.
Otherwise, any of the suggestions above sound reasonable to me. I’d probably just go with “Anonymous, via e-mail” myself.