Code for "F-U", as in "fuck you". Numbers are used to obscure the meaning: F is the sixth letter of the alphabet, U is the 20th. Can be used outright or hidden in the text of a message.
Dear Company Name
We love dealing with you. Sincerely Yours, Me. 6-20
Free Daily Email
Emails are sent from firstname.lastname@example.org. We'll never spam you.