interacts a bit oddly with bsd-mailx

[phlummox]

On Ubuntu, known to work with mailutils (e.g. mailutils 1:3.7-2.1, on Ubuntu 20.04).

But doesn't seem to work terribly well with bsd-mailx: things still get delivered fine, but attomail says no sender was supplied, so it'll make one up. (And the sender shows up as blank in the eventual maildir.)

(example: tried with bsd-mailx version 8.1.2-0.20180807cvs-1, from Ubuntu 20.04.)