User name get truncated if more then 20 character using Context.User.Identity.Name

I am facing a problem in my ASP.net application i am using Context.User.Identity.Name to send a email to user when the user name is more then 20 charater the user name get trancated to 20.
eg. ctw.mahesh.patil12345@ab.com
ctw.mahesh.patil1234@ab.com




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