So, body @ (a) munging, unmunged in original mail,
displays on web GUI: munged: postmaster(a)[127.0.0.1] postmaster(a)example.com unmunged: postmaster@localhost @example.com @localhost postmaster@localhost Posted unmunged from GUI, exact same as above.
And in the archive itself ugh, slightly worse, but not even the same set!: munged: <postmaster(a)localhost> postmaster(a)example.com postmaster(a)localhost unmunged: @example.com @localhost postmaster@[127.0.0.1] Posted unmunged from GUI, exact same as above.
And note that @ (a) munging is applied to body, not headers.
And I don't think Mailman 3 has an option to turn off munging.