Ok, works with some, and not with others tested - local / non-local doesn't seem to be a factor after all.
I wonder if forum emails haven't maybe been blocklisted somewhere on the net. May be worth checking, I know of two Forumites who have had their personal email addresses blocklisted by mistake so can easily happen to a larger concern.
As for my probable plan - a factor of gmail will redirect to Hetzner for easy access, which will forward to Vodamail to alert me as usual, which will then automatically get sucked up by standard gmail
Won't bore anyone with more detail