よく “–” で区切られれていますが、RFC 3676 的には最後に半角スペースを加えた “– " が正しいです。sig-dashes と呼ぶらしいです。
https://www.ietf.org/rfc/rfc3676.txt There is a long-standing convention in Usenet news which also commonly appears in Internet mail of using “– " as the separator line between the body and the signature of a message. When generating a Format=Flowed message containing a Usenet-style separator before the signature, the separator line is sent as-is. This is a special case; an (optionally quoted or quoted and stuffed) line consisting of DASH DASH SP is neither fixed nor flowed.
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