Every month or so I had this issue and I had to contact them through a form somewhere and I would get emails back from someone in india who reset my 'reputation'. They have some stupid made-up reputation system which means they need to see significant volume from you that is not marked as spam for them to accept your mailserver.

And yeah proton has similar issues. A lot of companies blackhole even confirmation emails there. So you can't confirm accounts with a proton email and they give zero indication as to why. Tinder and the internet archive (archive.org) come to mind.