This post is as much to help me remember as it is to help other people.
Below is a list (not comprehensive) of the postmaster resource pages for some of the major email providers.
| Microsoft / Outlook | https://postmaster.outlook.com |
| https://postmaster.google.com/ | |
| Yahoo | https://help.yahoo.com/kb/postmaster |
| Comcast | https://postmaster.comcast.net/ |
| AOL | https://postmaster.aol.com/ |
On these sites you can get information about how the provider handles spam, feedback loops, blacklists, whitelists, etc.
Very useful for those managing mailing list servers.