What Does RE: Mean in Email?
A comprehensive guide to what Re: means in email — from its Latin origins to how modern email clients handle it differently. Covers Re: vs RE: vs Fwd: vs FW:, why prefixes stack, whether Re: stands for 'reply' or 'regarding,' and the etiquette rules every professional should know.
The definitive guide to Re:, Fwd:, RE:, FW:, and every other email subject line prefix
Quick Answer
Re:in email means "Reply." It's automatically added to the subject line by your email client when you click the Reply button, indicating that your message is a response to a previous email. It helps both recipients and email software group messages into conversation threads.
There's a long debate about whether it comes from the Latin "re" (in the matter of) or simply abbreviates "reply." Either way, the function is the same: it marks a message as part of an ongoing conversation.
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Email Subject Line Prefixes Explained
Added by
Your email client when you click Reply
What it means
Indicates this message is a response to a previous email. It tells both the recipient and email software that this is part of an ongoing conversation.
Origin
The exact origin is debated. Some say it's from the Latin 're' meaning 'in the matter of' (as used in legal and business correspondence: 'Re: Invoice #1234'). Others say it simply abbreviates 'Reply.' RFC 5322, the internet standard for email format, defines it as a reply indicator without specifying what it abbreviates.
Example
Original: 'Budget Proposal' → Reply: 'Re: Budget Proposal'
Fun fact
The RFC standard says email clients SHOULD use 'Re:' (not 'RE:' or 'Re :'), but Outlook uses 'RE:' in uppercase. This inconsistency has confused email threading for decades.
Common Questions
Does 'Re:' stand for 'Reply' or 'Regarding'?
Both interpretations are valid, and the debate has been ongoing since email's inception. In email context, it functions as a reply indicator — your email client adds it automatically when you click Reply. However, in business correspondence (even pre-email), 'Re:' was used to mean 'regarding' or 'in reference to' (from the Latin 're'). The RFC 5322 standard that governs email format sidesteps the debate entirely — it defines 'Re:' as a reply indicator without specifying what it abbreviates.
Should I manually add 'Re:' to subject lines?
No. Your email client handles this automatically when you click Reply. Manually adding 'Re:' to a new email (not a reply) is misleading — it implies an existing conversation that doesn't exist. In cold email outreach, some people add fake 'Re:' to trick recipients into opening. This is deceptive, violates CAN-SPAM's prohibition on misleading headers, and damages trust.
Why do I see 'Re: Re: Re:' stacking up?
Most modern email clients are smart enough to collapse multiple Re: prefixes into a single 'Re:'. But when you mix email clients (Gmail, Outlook, Apple Mail), or when emails pass through mailing lists, the prefix-stripping logic can fail. Each client adds its own 'Re:' without realizing one is already there. It's cosmetically ugly but functionally harmless — the email still threads correctly based on headers, not the subject line.
Does changing the subject line break the thread?
It depends on the email client. Gmail primarily threads by subject line, so changing it usually creates a new thread. Apple Mail and Thunderbird thread by headers (In-Reply-To and References), so changing the subject keeps the message in the original thread. The safest approach: if you want to stay in the thread, don't change the subject. If you want to start a new conversation, compose a fresh email rather than replying and changing the subject.
What does 'Re:' mean in non-email contexts?
Outside of email, 'Re:' (or 'RE:') typically means 'regarding' or 'in reference to.' It's commonly used in legal documents, business memos, and formal letters: 'Re: Invoice #456.' This usage predates email by centuries — it comes from the Latin ablative of 'res' (thing/matter). In email, it evolved to primarily indicate a reply, though the 'regarding' meaning still colors how people interpret it.
How Different Email Clients Handle Prefixes
| Client | Reply Prefix | Forward Prefix | Stacks? |
|---|---|---|---|
| Gmail | Re: | Fwd: | No — collapses to single Re: |
| Outlook (Desktop) | RE: | FW: | Sometimes stacks RE: RE: |
| Outlook.com | Re: | Fw: | No — collapses |
| Apple Mail | Re: | Fwd: | No — collapses |
| Thunderbird | Re: | Fwd: | No — collapses |
| Yahoo Mail | Re: | Fwd: | Can stack across clients |
The History of "Re:"
Pre-email (centuries ago):"Re:" was used in legal and business correspondence meaning "in the matter of" — from the Latin ablative of "res" (thing, matter). A lawyer might write "Re: Estate of John Smith."
1970s-80s (early email):When email systems needed a way to indicate replies, they borrowed the existing "Re:" prefix. Whether early implementers meant it as "reply" or "regarding" is lost to history — probably both.
1982 (RFC 822):The first email standard defined "Re:" as a reply indicator without specifying what it abbreviates. This deliberate ambiguity continues in the current RFC 5322 (2008).
Today:Most people interpret "Re:" as "Reply" in email context. The Latin "regarding" meaning lives on in formal correspondence, legal documents, and the occasional pedantic internet debate.
Etiquette Tips for Email Prefixes
- Never manually add "Re:" to a new email — it's misleading
- If a thread drifts to a new topic, start a fresh email with a new subject
- Don't remove "Re:" from replies — it breaks threading in most clients
- If you see "Re: Re: Re: Re:" stacking up, it's a client mismatch — harmless but ugly
- Use "Fwd:" honestly — forwarding without the prefix hides the email's origin
About this tool
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.
Related tools
- Email Thread Explainer — Deep dive into how email threading works
- Email Header Analyzer — Inspect the headers that control threading
- Subject Line Tester — Optimize your email subject lines
- Email Abbreviation Reference — 70+ email terms explained
- Email Sign-Off Generator — Professional email endings
- Professional Email Phrase Generator — Better alternatives to overused phrases
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