Imposing a rate limit on an application is a sure fire way to limit how many users an app can have in the app store. What's the point of having a third-party email app, if you are not allowed to connect to Gmail?

As the FairEmail author points out, this is plain out anticompetitive behaviour from Google, since their own Gmail app doesn't suffer from any limits.

Some other company I would give the benefit of the doubt, but in Google's case I assume a pure malice. Are they jealous that one opens source developer was able to provide a better app than Gmail, although they have practically unlimited resources?

The developer regularly publishes updates on the topic: https://email.faircode.eu/status/

I would like to see the EU put a stop to Google's anti-competitive behavior. This is a textbook example of why more regulation is needed.