I can confirm that this behaviour is still present in Evolution 2.12.3 (evolution-exchange version 2.12.2.dfsg-1+b1). The exact text of the error message is:
Could not configure Exchange account because an unknown error occurred. Check the URL, username, and password, and try again. Regards, CC -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
