Report it with relevant authorities.

This is anti-competitive behaviour.

Especially clear since, as GP points out mail gets through if it is a direct reponse, meaning they knew very well it was a legitimate address.

This is abuse of bulk mail filter to squeeze out local competitors.

That sounds like a stretch even by EU standards when a acting openly envious. Google can warn or ban their misbehaving users for repeated spam violations. They can't ban people on other people's domains. "Legitimate addresses" can still send spam and it could have been caused by ignorant users who think "junk" is for non-immediate deletion and use it to keep their inbox tidy. In fact they /did/ deliver it but prefiltered.

Even if they could kangaroo court it for standard business practices they wouldn't since they don't want to be known as "the dumb fuckers who banned domain based spam filtering" and face pressure from annoyed constituents now receiving piles of spam.

> "Legitimate addresses" can still send spam

It seems clear from the description above that poster is someone recipient wants mail from.

As for the ignorant users using spam button I've seen that once since I started working.

> Report it with relevant authorities.

Happy to do so, but which are the relevant authorities?

I have lived in Norway more or less my entire adult life so I don't really know about UK but here it is konkurransetilsynet.

I tried searching for it though and this seems relevant:

https://www.gov.uk/government/organisations/competition-and-...

You local antitrust regulator.