It is commonly known that when using SQL Server with Active Directory you need to utilize the DOMAIN\UserName format and that the UserName cannot be more than 20 characters. If you do use a name with more than 20 characters then the name gets truncated and you are left with only a partial name.
Just today I stumbled upon an old (2009) Microsoft Connect suggestion which is still available for votes. At this time Microsoft does not plan on supporting the UPN format (email@example.com) which does allow more than 20 characters.
I recommend that everyone takes the time to follow the link below and vote for this suggestion. Maybe if we get enough people over time Microsoft will decide to support these types of accounts.