It's an informal standard going back a long way. See for example RFC 3676, 4.3. Usenet Signature Convention There is a long-standing convention in Usenet n

It's an informal standard going back a long way. See for example RFC 3676, 4.3. Usenet Signature Convention 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. Generating agents MUST NOT end a paragraph with such a signature line. A receiving agent needs to test for a signature line both before the test for a quoted line (see Section 4.5) and also after logically counting and deleting quote marks and stuffing (see Section 4.4) from a quoted line. * https://www.rfc-editor.org/rfc/rfc3676.txt https://www.rfc-editor.org/rfc/rfc3676.txt