I just installed EM Client on Windows. In the “Incoming Server” page, I ticked “IMAP”, and in “Incoming Server” I typed in an IPv6 IP number. I tried both as is, and prefixed with “[” and postfixed with “]”. I also tried appending “:” and a port number.
In all these situations, when I click Next, I get an error box “Cannot parse the port number you specified”.
Supporting IPv6 IP number is a fundamental function.