Re: Meaning in Email
In email reply subjects, “Re:” is the conventional marker showing that the message concerns the original subject. RFC 5322 explicitly describes it as an abbreviation of Latin “in re,” meaning “in the matter of,” and recommends a single instance rather than repeatedly stacking “Re:” prefixes.
In an email subject, “Re:” normally marks a reply about the same matter. RFC 5322 traces it to Latin “in re,” meaning “in the matter of”; it is not simply the prefix “re-” meaning “again.”
Key details
Important caveats
Threading Boundary
RFC 5322 says a reply subject may begin with one literal “Re: ” followed by the original subject and cautions against stacking multiple instances.
Further guidance
Manual Reference Note
Outside automatic reply subjects, business correspondence also uses “Re:” to introduce the matter or reference being discussed; the practical idea is “regarding / in the matter of.”
Meaning
In a reply subject, “Re:” marks the message as a response concerning the original subject. It should not be interpreted as the English prefix “re-” meaning “again.”
Practical Application
In an email subject, treat “Re:” as the conventional reply marker for the same matter rather than as the ordinary English prefix “re-” meaning “again.” Let the mail client preserve it when replying unless house style or workflow rules require another subject.
Reader Check
Check whether “Re:” was generated as part of a reply thread or typed as a new subject label. Do not expand it to “regarding” as though that were its literal origin when explaining the convention.
Worked Example
Original: “Subject: Budget review.” Reply: “Subject: Re: Budget review.” The reply marker keeps the message associated with the same subject thread.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- RFC 5322 — Internet Message Format (opens in a new tab)RFC Editor · Email Subject-field convention for Re: in replies and its “in re” / “in the matter of” meaning
- Business letters: Reference line (opens in a new tab)Government of Canada Language Portal · Canadian government guidance for Re: reference lines in business letters