How do modern email systems try to resist connection$-$based attacks?

Question $79$ options:

Most servers and clients use SSL$/$TLS$.$

Email services discourage connection$-$based services like POP$3$ and IMAP in preference to web$-$based email service.

Modern email client protocols incorporate more advanced security measures, like challenge response authentication, at the application level to resist all connection$-$based attacks.

Email systems maintain blacklists of dangerous internet hosts and block connections with those hosts.