Enter a clue or pattern or both:
The Clue
E-mail add-on (3 letter answer)
The Answer
BCC
Tags
Notes
The answer BCC is rarely seen, appearing only about once every 2100 puzzles.
Related Clues
Include surreptitiously, in a way
Abbr. at the top of an email
Secret attachment, for short
Abbr. atop some e-mails
Abbr. on top of some e-mails
Letters in front of an e-mail address