When written communications were usually delivered on paper, the term re meant "concerning" or "with regard to." It was used at the top of a formal letter, followed by the subject of the letter. Re is not an abbreviation. Rather, it is derived from the Latin in re, meaning "with regard to."
How to Reply to Email Inline
In electronic communications, however, re is used again. RE: In the subject line of an e-mail, precedes the subject when the message is a reply to an earlier message that used the same subject.
This indicator allows you and your correspondents to recognize messages and replies that are on a specific topic. This is useful if you are working on multiple email conversations at the same time.
RE: is usually automatically placed before the subject of a reply email. However, if you manually place it in a new message, meaning "regarding," recipients may be confused. They may think the message is a reply to an email thread they haven't seen. Regardless of what may be true in other contexts, RE: or Re: does not mean "regarding" in email correspondence. It means "reply."
