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.”
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