This is not "gmails" behavior. This is called "verb" and it is one of the best things ever. I only use verb for signing up at stuff. So, i can sort, and know when someone misuses my mail address.
And, if i would need to create an alt, to post stuff, that i don't want somebody do bring in connection with me (TIFU, NSFW, whatever you like) i would use verb.
First of all: I'm posting this from my .ml alt. Because i can not do it from my .world main. That i can't do it, i found out just because i was waiting for a response on a comment where is was sure that the OP would respond. After searching, i found out that my comment and my DM's never where federated to .ml.
So, that said: I'm all for defederating bad instances, i'm all for separation where it makes sense. BUT:
If an instance is listed on join-lemmy, this should work as the normal user would expect
We are not ready for this yet. We are missing features (more details below)
Even instances that officialy require applications, can be spam instances (admins can do what ever they want), so we would need protection against this anyways. Hell, one could just implement spam bots that talk directly federation protocol, and wouldn't even need lemmy for this ...
Minimal features we need:
Show users that the community they try to interact with is on a server that defederated the users instance
Forbid sending DM's to servers that are not fully federated
Currently, all we do is: Make lemmy look broken
And before someone starts with: "Then help!", i do. I do in my field of expertice. I'm a PostgreSQL Professional. So i have build a setup to messure the lemmy SQL performance, usage patterns, and will contribute everything i can to make lemmy better.
(I tried rust, but i'm to much C++ guy to bring something usefull to the table beyond database stuff, sry :( )