> When used in a reply, the field body MAY start with the string "Re: " (an abbreviation of the Latin "in re",
RFC 5322 に言及があるレベルで本当松なのか
RFC 5322 - Internet Message Format
https://datatracker.ietf.org/doc/html/rfc5322#section-3.6.5
IETF DatatrackerRFC 5322: Internet Message FormatThis document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK]
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