octal escapes with rmail [was: Re: dash.el [was: Re: Imports / inclusion of s.el into Emacs]]

[Alfred M. Szmidt]

   P.S.  Would you please fix your MUA to not mangle non-ASCII names of
   your correspondents?  They come out as octal escapes, for some
   reason.  Maybe it's because your message doesn't state an encoding.

Thank you for the hint. I'm using rmail (and Emacs 26.3), maybe
someone who knows rmail here can pitch in why it might be doing that
or where to look.