Libravatar is a federated avatar hosting, similar to Gravatar but decentralized.
It works by querying _avatars._tcp SRV record and pulling the avatar from the discovered avatar service based on the hash of email address.
I do not propose to pull avatars for other users from Libravatar, as it leaks hashes of email addresses you query to avatar service, which in turn can be easily matched to actual emails. But a single pull of avatar for your own address during autoconfiguration should not hurt, as you will connect to your own email server anyway. This way you don’t need to manually configure your avatar if you already run a self-hosted email server with Libravatar support.
For privacy reasons, make sure that the address SRV record points to is a subdomain of the server address and ignore the cdn.libravatar.org fallback.