I also enable dovecot logging (at both info and debug levels) so I can also watch login attempts. You can configure it to also log failed login attempts along with passwords if necessary.
A flat out connection failed error denotes firewall or iptables problem. iptables on CentOS has been deprecated in favour of firewall-cmd which is a royal pain (at least for me) to get one’s head round, but you can still use iptables by reenabling it if preferred. Lots of info online for that.
Do you get an error if you connect with a client on your own PC?