MIME Headerって1行の長さ上限が無かったっけ。古い話？
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んー……？998は絶対越えちゃだめ、78はできるだけダメ(?)、くらいかな。
https://datatracker.ietf.org/doc/html/rfc2822#section-2.1.1
> Each line of characters MUST be no more than 998 characters, and SHOULD be no more than 78 characters, excluding the CRLF.
RFC6532で998制限にまとめてるのかな。
https://datatracker.ietf.org/doc/html/rfc6532#section-3.4
> Section 2.1.1 of [RFC5322] limits lines to 998 characters and recommends that the lines be restricted to only 78 characters. This specification changes the former limit to 998 octets.
そして文字じゃなくてオクテットになった、と。
> (Note that, in ASCII, octets and characters are effectively the same, but this is not true in UTF-8.)