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What Does RE: Mean in Email?

Re: in an email subject line means the message is a reply. Your email client adds it automatically. Here is what it stands for and why the prefixes stack up.

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What does Re: actually mean?

"Re:" in email indicates a reply. When you click the Reply button, your email client automatically adds "Re:" to the beginning of the subject line. This tells both the recipient and their email software that your message is a response to a previous email.

The origin of "Re:" is debated. Some say it comes from the Latin "re" meaning "in the matter of" (used in legal correspondence for centuries). Others say it simply abbreviates "Reply." The RFC 5322 standard — the internet specification that governs email format — defines it as a reply indicator without specifying what it abbreviates. So technically, both interpretations are correct.

Re: vs. RE: vs. Fwd: vs. FW:

Different email clients use different formatting for the same prefixes:

  • Re: — Used by Gmail, Apple Mail, Thunderbird, Yahoo. The RFC-standard format.
  • RE: — Used by Microsoft Outlook (desktop). Same meaning, different case.
  • Fwd: — Used by Gmail, Apple Mail, Thunderbird for forwards.
  • FW: — Used by Microsoft Outlook for forwards.

These differences are purely cosmetic but can cause stacking (Re: RE: Re:) when messages pass between different email clients.

Common misconceptions

  • "Re: means Regarding" — In traditional business correspondence, yes. In email, it specifically means Reply. Using "Re:" to mean "regarding" in a new email is technically incorrect and can confuse threading.
  • "Adding Re: makes emails look important" — Some marketers manually add "Re:" to cold emails. This is deceptive, violates CAN-SPAM, and damages trust.
  • "Re: Re: Re: means my client is broken" — It's just a formatting issue from mixing email clients. The thread still works correctly.

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Frequently Asked Questions

What does Re: mean in email?

Re: in email means 'Reply.' It's automatically added to the subject line by your email client when you click the Reply button. It indicates that your message is a response to a previous email and helps email software group messages into conversation threads. The prefix comes from either the Latin 're' meaning 'in the matter of' or simply abbreviates 'Reply' — the RFC email standard deliberately doesn't specify which.

Does Re: stand for 'reply' or 'regarding'?

In email context, it functions as 'Reply' — your email client adds it automatically when you reply. However, in traditional business correspondence (pre-email), 'Re:' meant 'regarding' or 'in reference to,' from the Latin ablative of 'res' (thing/matter). The RFC 5322 standard defines it as a reply indicator without specifying what it abbreviates, so both interpretations have some validity. For practical purposes, treat it as 'Reply' in email.

Why does Outlook use RE: instead of Re:?

It's simply a formatting choice by Microsoft. The RFC email standard recommends 'Re:' (with only the R capitalized), but Outlook has always used 'RE:' (all caps). Both mean the same thing. The difference occasionally causes cosmetic issues — when emails pass between Gmail (Re:) and Outlook (RE:), you might see 'RE: Re: RE: Topic' because each client adds its own preferred format without recognizing the other's.

What's the difference between Re: and Fwd:?

Re: means you're replying to someone in the existing conversation — your message stays in the thread. Fwd: means you're forwarding the email to someone new who wasn't part of the original conversation. The key technical difference: replies include In-Reply-To headers that maintain the conversation thread, while forwards do not — the forwarded message starts a new thread for the new recipient.

Should I manually type Re: in a subject line?

Never. Your email client adds Re: automatically when you click Reply. Manually typing 'Re:' in a new email (not an actual reply) is misleading because it implies an existing conversation. Some cold emailers use this trick to boost open rates, but it violates CAN-SPAM's prohibition on misleading headers, damages trust, and increases spam complaints. If you want to reference something, use 'Regarding:' or 'About:' instead.

Why do I see Re: Re: Re: Re: stacking up?

This happens when emails pass between different email clients. Modern clients like Gmail collapse multiple Re: prefixes into a single 'Re:'. But when a Gmail user (Re:) replies to an Outlook user (RE:) who replies back, the prefix-stripping logic can fail because each client uses a different format. It's cosmetically ugly but functionally harmless — the email still threads correctly based on headers (Message-ID, In-Reply-To), not the subject line prefix.

Does removing Re: from the subject break the email thread?

It depends on the email client. Gmail threads primarily by subject line, so removing Re: may break the thread. Apple Mail and Thunderbird thread by Message-ID headers, so removing Re: from the subject has no effect on threading. The safest approach: don't manually modify subject lines in replies. If you want to change the topic, start a completely new email (Compose, not Reply) to create a clean break.

What does Re: mean outside of email?

Outside email, 'Re:' means 'regarding,' 'in reference to,' or 'in the matter of.' It comes from the Latin ablative of 'res' (thing/matter) and has been used in legal and business documents for centuries. You'll see it in legal briefs ('Re: Case No. 2024-1234'), business memos ('Re: Q3 Budget Review'), and formal letters. In these contexts, it clearly means 'regarding' — the 'reply' meaning is specific to email.

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