I think, at the least, the documentation should state that X-Message-SMTP-Method can't be used with smtpmail queued delivery.

On Fri, 23 Jul 2021 at 3:06 pm +0200, Lars Ingebrigtsen wrote:

Rajeev N via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org> writes:

> [Quoted text removed due to X-No-Archive]

X-Message-SMTP-Method is a Message thing – the mail backends (smtpmail, sendmail etc) don't know anything about it (and it's removed before it reaches smtp.el).

So I don't think this is a bug?