"Email this user" is available from the "toolbox" on the left of the screen, when the User or User talk page of any editor with email enabled is viewed. This page sets out information on the Special:EmailUser feature, a MediaWiki feature that allows registered users to exchange emails with other registered users privately ("off-wiki").
Users can send emails to other users via the built-in email facility on Wikipedia and any of its sister websites. Both the sender and recipient must have provided email addresses in their preferences. The sender's username and email address are disclosed but not their IP address. (Note that replying "the wrong way" can disclose personal information—see Privacy issues below for how to avoid this if needed, when you reply).
Discovered by embedding cosine similarity (sentence-transformers MiniLM, 384-dim).