I think you tested by sending your test mail with your @myyuonohostdomain.tld email to contact@domain.tld.
So the yunohost instance evaluate that gandi send a mail with a sender in @myyuonohostdomain.tld… And it don’t like this cause your instance is the only one authorized to send mail in xxx@myyuonohostdomain.tld.
OR maybe you added the domain.tld to your yunohost ?