Reads mail while leaving it on the server, keeping folders and read state consistent across every device. The sensible default for modern mail.
Plain text unless STARTTLS is negotiated. Use IMAPS on 993 where you can.
Open 143 is fine if STARTTLS is enforced and plain-text logins are refused. If it accepts an unencrypted login, fix that first, then move clients to 993.
The port scanner will tell you whether 143 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.