In its modern usage, ‘Re:’ is written at the beginning of an email subject line to indicate that a reply has been sent to that email. It’s a useful prefix to include if you are corresponding with multiple people at once or with one person across different emails. Because of the ‘Re:’ prefix, everyone knows which email is being referred to and who has replied to what.
You should only use ‘Re:’ in the subject of the email when replying to an existing message. Centuries ago, ‘Re:’ was used at the top of formal letters as a subject line of its own. It came from the Latin ‘in re’, which means ‘in the matter of,’ so it was correct to use it when sending letters even if it wasn’t to reply.
However, the way it’s used in modern-day email correspondence is to indicate a reply, so any alternative use may cause confusion. Here’s an example of proper use and improper use of Re: in an email:
- Re: July marketing strategy
- Re: Hello, Jim. I wanted to speak with you about the new marketing strategy for July…
The quicker and tidier the subject line is, the more easily ‘Re:’ can be used to indicate an ongoing conversation and the subject matter, rather than a confusing line of text.