At my company we are using dockerized mailcow on a Hetzner VPS and it has taken us some time to have the IP whitelisted in all major e-mail providers.
The easiest to work with have been Microsoft and Yahoo. I still haven't found a way to whitelist our IP on centurylink.net, charter.net and att.com (please let me know if you have any ideas)
Some links that might be of help to others:
[1] https://sendersupport.olc.protection.outlook.com/snds/data.a...
[2] https://io.help.yahoo.com/contact/index?page=confirmation&lo...
[3] https://postmaster.google.com/u/0/dashboards#st=userReported...
[4] https://support.microsoft.com/en-us/supportrequestform/8ad56...