What characters/length are supported in Mail.app IMAP passwords? I use a password manager and created the password LX7mQW9Tw^V0$Ef2Ag#v*
    (* don't worry, that's not my password any more, keep reading)

My DNS / email provider accepted it and I could use it successfully to log in on their webmail app.
I followed their document on configuring (basic TLS-wrapped IMAP &amp; SMTP) in Mail.app and kept running into trouble.   Eventually, while verifying my password I got the bright idea to change it to something shorter and with less-special special characters.   Finally, as the climax to about an hour of troubleshooting, it worked!
Now I want to add a note for account setup to help my people avoid this limitation in, apparently, Mail.app (or not, but that's all the evidence I have so far).
So back to the question in the topic:  Are there known limitations as to what characters or length Mail.app will support for IMAP accounts?
Thank you!
I have longer IMAP and SMTP email passwords than that, using all of those special characters (except maybe $) and have no problems using Mac OS X Mail.app
While it's possible that one of those characters caused the issue, I think that's more likely on the IMAP server side.
As far as I am aware there's nothing in OS X Mail.app which should have prevented that password from working. I know it handles passwords that are 50 characters long and contain a variety of special characters just fine.
If you're concerned, try changing the password again, and if you run into problems try adding length to the password and using letters instead of special characters. Length generally beats complexity for password entropy anyway.