Maybe this is linked to this topic: https://github.com/NethServer/dev/issues/7433 ?
But not actually sure, because if trying the workaround from here: https://community.nethserver.org/t/single-relay-for-account-does-not-work-but-in-a-strange-way/25575/7, if I enforce the 'sender/login match, both user 1 and user 2 cannot send at all (from webtop failure: "553 5.7.1 : Sender address rejected: not owned by user USER2", from other mail client failure "<user1@domain.a>: Sender address rejected: not owned by user user1 .