Not a question, but in case anyone has this problem… I had an email account set up on a custom domain, and suddenly, MacMail was telling me that the username/password could not be verified. This has been a minor pain in my ass over the years, usually requiring me to go into the panel and re-entering the (same) password, and then it was ok.
Not this time.
I wound up opening a ticket, and got relatively useless advice, so I dug some more - and it seems that recently, using mail. for the names of the mail servers was not correct anymore. I followed the directions in the “setup your mail client” page, found the correct new server names (imap.dreamhost.com and smtp.dreamhost.com, but it will likely be different for you), and all is now well and my girlfriend will talk to me again.
So - if you have weird mail problem, check the server names.