1. no 2fa in the webmail yet. work is currently underway at the ietf for standardizing chained SASL (auth) mechanisms, and passkeys. i want to look at implementing passkeys already for the web interfaces, but there is much more on the todo-list...
2. i as shown by yamrzou, i don't think so. SASL auth really just uses 1 auth mechanism at the moment. i think there is also standardiziation work underway for password+totp sasl authentication. but clients (like thunderbird) would still have to implement it before it's useful. there may be a trick to get 2fa-like authentication now, using both TLS client cert authentication (mox supports this based on public key identification, no other properties of certs) and a IMAP/SMTP-level SASL authentication.
3. no, but this is interesting. what kind of rules would you set? rules to match specific message headers/content that identify a message a phishing and reject it? for when attackers send the same message to many employees? do you need to remove messages from their inboxes after it has been delivered (assuming all employees would get the email at around the same time)?
4. no, but i've considered adding it. it should be very simple to add. and it's much better than mail servers modifying the message content to add messages like that.