I too have been having this happen to two of my email addresses, both using the same domain.
First one started over a week ago, the other yesterday,
Bizarre thing is that it is not rejecting all messages. The bulk that are rejected are emails containing lots of 'non-words', which is understandable, but some of those messages get through.
Then again, other 'normal' messages are sometimes being rejected but not always.
I have tested this by sending messages direct to those addresses, as well as via groups.io, so I presume my email provider uses spambot, which is a pain in the arse.
For what it may be worth, I think ALL server side spam filters are a bad idea. What one person may think is spam, another does not, and once you get on these blacklists it can be a pain to get off them, plus nothing is to stop you getting black listed again.
Personally I think all spam filtering should be done by the user.
As for groups.io, well it is a mailing group, and whilst I realise a lot of people read and post via a browser, it a MAILING group after all, so I am sure the majority of people read and post using their email software, so local spam filtering comes into play.
Just my penny's worth