To see what IPs have been blocked by Fail2ban type the following
iptables -L -n
This will output the iptables list and at the bottom you will see the Chain fail2ban-dovecot-pop3imap
Chain fail2ban-dovecot-pop3imap (1 references) target prot opt source destination DROP all -- 18.104.22.168 0.0.0.0/0 RETURN all -- 0.0.0.0/0 0.0.0.0/0
Here you can see that 22.214.171.124 has been blocked by Fail2Ban when they attempted to brute force attack dovecot. If for some reason this host is not to be blocked, you can remove the IP 126.96.36.199 from the block list by using the following
iptables -D fail2ban-dovecot-pop3imap -s 188.8.131.52 -j DROP
This will remove the host from the block list and they should be able to access you server again.