Slow password guessing for imaps / 2018-06-22

Interesting in the logs:

Jun 19 21:22:29 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.9]
Jun 19 21:23:30 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.9]
Jun 19 21:27:05 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]
Jun 19 21:31:58 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]
Jun 19 22:27:15 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.9]
Jun 19 22:30:10 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.9]
Jun 19 22:44:17 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]

..

Jun 22 14:23:39 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]
Jun 22 14:24:35 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]
Jun 22 15:20:05 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.9]
Jun 22 15:21:01 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.9]
Jun 22 15:29:18 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]
Jun 22 15:30:06 greenblatt imapd-ssl: LOGIN FAILED, method=PLAIN, ip=[::ffff:5.188.207.11]

Every time fail2ban blocks the addresses for a while but the attacker is more persistant than that.