I’m just curious if ‘B’ still retrieves the content from ‘A’ to show in user feeds.
It works the other way around: instance A pushes the content to instance B. Therefore if A defederates B, then obviously A ain’t gonna be pushing the content.
There’s an edge case where instance C is involved: A could comment on a post on C, and then C would forward it to B as well. But then B wouldn’t be allowed to fetch the user profile from A anyway and might just drop it regardless. I’m not sure the particular way Lemmy handles this.
It’s not impossible, been running my own email server for about 10 years and I inbox pretty much everywhere. I even emailed my work address and straight to inbox. I do have the full SPF, DKIM and DMARC stuff set up, for which I get notices from several email provides of failed spoof attempts.
Takes a while and effort to gain that reputation, but it’s doable. And OVH’s IPs don’t exactly have a great reputation either. Once you’re delisted from most spam databases / old spam reputation is expired, it’s not that bad.
Although I do agree it’s possibly one of the hardest services to self host. The software to run email servers is ancient and weird, and takes a lot to set up right. If you get it wrong you relay spam and start over, it’s rough.