Let's talk about Microsoft as serious mail provider: I self-host my mail server for <10 years. My server is in 0 blocklists. DKIM / DMARC / SPF implemented since ages. The company I work for, hosts their emails on 0365, I have been sending emails from one account to the other with no problem. Recently I got an email from an outlook.com account. I am in the contacts list of the sender. I tried to respond, my email was rejected. The email auto-response from MS contained links with "things you should do in order to make mails from your server acceptable by MS". I have everything done already since ages. Nothing missing. I googled on how to "appeal". I find a hell lot of people complaining in MS forums about it, no obvious solution.

Eventually I find a portal from MS to do exactly that: Appeal. I tried to use the portal but I got an error "event service error" - or something similar. Portal not working. I say "what the heck - I ll try later". I do try later, same outcome. Several times. For days. I google the error, I find several threads on redit since years complaining that the portal doesn't work giving this exact error. Since years. I keep trying nevertheless to respond to the email and go through the portal again for days. Nothing. I try the following: I signup for azure, it asks me for an email address, I say I have none and guides me to sign up to outlook. Outlook asks me for a backup email address, I provide my email address from the blocked server.

I then try to respond again to the original email that outlook has been rejecting - WORKS. Summary again: MS blocks email my mail server for not fulfilling requirements that it actually fulfills. MS portal to object, doesn't work since years. I create an outlook email and set self-hosted mail address as "backup" - my server gets unblocked.

Fun fact: I received "on-boarding" emails from MS Azure to my new MS Outlook account. MS Outlook classified them as "Junk".

Does this qualify as a "serious mail provider"?

--updated wording.