You seem to be using a "telegraphic"style - omitting words that aren't necessary for full understanding. If so, you can just write:
User able to send mail. (Do you mean "email"? If so, I'd use that.)
Use can send mail.
If you want to write a full, proper English sentence, then either of these is fine:
The user is able to send mail.
The user can send mail.
Student or Learner