Comment by alexjplant
10 hours ago
When I ran my own mail server Microsoft was the only company I encountered that would black hole my messages - no SMTP error for my own server to bounce back to me, no bounce back from their server, nothing. I vaguely recall having to do a dance with them a few times to fix this and the last time I tried I received no response. I don't frequently interact with Office 365 users so this didn't matter much to me.
I did end up later moving to Proton primarily out of laziness. I thought these issues would be a thing of the past until I applied to work at a company that administered their own Exchange server that also black holed my messages from Proton's servers. Their reasoning? "We geo-block Switzerland for security reasons." Needless to say I turned them down.
Oh when I ran my own mailserver I did get SMTP errors back.
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.