The Meaning of RE: in Emails

RE: has different definitions in paper and electronic communications

When written messages were commonly delivered on paper, the term re stood for "regarding" or "in reference to." It was used at the top of a formal letter, followed by the subject of the letter. Re isn't an abbreviation. Rather, it's taken from the Latin in re, which means "in the matter of."

With electronic communications, however, re has been repurposed. RE: in an email subject line precedes the subject when the message is a reply to a previous one that uses the same subject.

This indicator helps you and your correspondents recognize messages and responses that are on a particular topic, which is helpful if you're engaged in several email conversations at the same time. 

When RE: Causes Confusion in Emails

RE: is typically added in front of the subject of a reply email automatically. However, if you put it there manually in a new message, meaning to indicate "regarding," recipients may be confused. They may think the message is a reply that belongs to an email thread they haven't seen. Regardless of what might be true in other contexts, in email correspondence RE: or Re: doesn't mean "regarding." It means "reply."

It's common for cold-solicitation business-to-business emails to start with a re to exploit this ambiguity. People savvy with Outlook rules sometimes filter as spam any messages that start with re: when no previous message exists.

To prevent confusion, don't insert RE: in a subject line. The fact of the subject line itself implies that this is what the message is "regarding." Instead, allow RE: to appear when you reply to an email message sent to you.