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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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.

What Does "Re:" Mean in Email?

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

Re:

Reply (or Latin: 're' — in the matter of)

▲

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.

RE:

Reply (Microsoft Outlook format)

▼

Fwd:

Forward

▼

FW:

Forward (Microsoft Outlook format)

▼

Fw:

Forward (alternative format)

▼

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.

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