Emacs message message-mark-inserted-region
Cut Here From the bygone era preceding Markdown and Stack Overflow, emacs’ message mode developed a marker for code areas within an email. With message-mark-inserted-region, by default in a message with a selection C-c M-m, you get something like this:
--8<---------------cut here---------------start------------->8--- Secure Connection Failed An error occurred during a connection to example.com. Peer’s Certificate has been revoked. Error code: SEC_ERROR_REVOKED_CERTIFICATE The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.