Okay, so only after 3 failed attempts at installing enigmail did I look up “enigmail Thunderbird” in search engines.
Once I had I2P properly running, I got me a mail account from hq.postman.i2p. Accounts can send and receive mail from the normal internet. Postman’s I2P-based mail system can be accessed either via I2P’s built-in mail client (susimail) or using any mail client that supports smtp and pop3. But not all clients are suitable for that.