My email address is a mac.com address which is also my AppleID. After upgrading to 10.11 new email created in Mail.app use the me.com variation of my address! When I look in the iCloud preferences, icloud.com, my iPhone, Yosemite macs and the apple ID management site my email address ends mac.com but not on 10.11.
When I look at the Account Info/Summary from Mail my email address is listed as me.com Where is this coming from?
I've tried deleting all of the mail caches and preferences etc from ~/Library and restarting Mail but it still has the wrong email address. I've never used the me.com cariant of the address. The bizzare thing about this is that me.com is superceded by icloud.com so I could sort of understand if there was some glitch that was never tested with the really old me.com addresses and replaced them with icloud.com but going to the intermediate me.com variant?
I should point out that the result of this is that it is impossible to send an email from the updated Mac using my mac.com address since me.com is the only option recognised by mail. In the Mail preferences under accounts the email address is uneditable and there is nothing on the alisa pulldown menu.