Re: (in Email)
The prefix mail clients automatically add to a subject line when you reply to a message.
Definition
Re: is the short prefix that email clients automatically prepend to a subject line when you reply to a message, producing something like "Re: Your order confirmation." The letters come from the Latin phrase "in re," meaning "in the matter of," and were used in formal correspondence long before email existed. What does re mean in an email is a genuinely common question, because most people assume it simply abbreviates "regarding" or "reply," which is a reasonable but technically incorrect reading of an older Latin convention.
Why It Matters
Re: signals to a recipient (and to their mail client's threading logic) that a message is part of an ongoing conversation rather than a new one. Mail clients use it, along with the original subject line and message headers, to group replies into a single visible thread. In cold outreach and marketing email, keeping or dropping the Re: prefix affects whether a message reads as a genuine continuation of a conversation or as a first contact.
How It Works
When you click reply in a mail client, the client copies the original subject line and adds "Re: " to the front if it isn't already there. If you reply again to that same thread, most clients recognize the existing Re: and don't stack a second one, so a well-behaved thread shows "Re: Original Subject," not "Re: Re: Re: Original Subject." Message headers (like In-Reply-To and References) do the real work of threading behind the scenes; the Re: prefix is a visible convention, not the mechanism itself.
Best Practices
- 1Let your mail client or platform add Re: automatically rather than typing it manually
- 2Don't strip Re: from a genuine reply just to make an email look unopened or new, since it can read as manipulative if noticed
- 3Avoid manually adding Re: to a cold outreach subject line to fake an ongoing conversation; this is a known deliverability and trust red flag
- 4Use Fwd: (not Re:) when forwarding a message you didn't write, to accurately signal what the recipient is receiving
- 5Keep the original subject text after Re: intact so threading stays accurate across clients