Forwarding of CiviMail to addresses on same hosting service fails
The hosting company mail server was objecting to VERP mailoxes like
Return-Path: b.544.88425.a5cada51f4f1fa94@mydomain.com
that are not prefixed with a real mailbox, treating them as unknown addresses and discarding the post. The discarding is noted in the logs, which are not available to me.

There is a setting CiviCRM > Adminster > CiviMail > Mail Accounts
in which "Localpart" of the Bounce record needs to be a host mailbox followed by "+" e.g "reply-mail+".
(There is also a setting "Return-Path" which seems to be ignored, at least if VERP is being used.)
Localpart gets prefixed to the VERP addresses to make, e.g.
Return-Path: &lt;reply-mail+b.578.89179.a1e28883fd23b0e3@mydomain.com&gt;
and the addresses are no longer seen as unknown and the posts are sent on their way. Delivery was ultimately successful to Gmail apps mail (where it usually went to the Promotions tab) and to other services.

This post is pertinent.