I have encountered an issue with a certain email account that required the username to be the actual email address. By default on Yosemite (issue could be found on previous versions of OSX), the username of the account gets truncated upon creation. This wouldn't be a problem if Mac Mail was designed with the ability to edit email account information, but it does not. The username line that we need to edit is grayed out.
You have 2 options to repair this annoyance:
1. Remove and re-add the account again through Mac Mail preferences -> accounts with the correct username, not Mac Mail -> Accounts
2. (More fun way) Manually edit the plist located in /Users/yourusername/Library/Mail/V2/MailData/Accounts.plist
change your username under where it says username ;-)
This could also work if your username has changed or otherwise is in need of editing.
