Let's start with the Webmail MTA hostname. Why do you have that entry in there? According to your DNS records it points to a different IP address than your MX records does, is there any reason for that and do you have multiple mail servers?
It's really not a good idea (i.e. it's a highly insecure setup) to be using POP3 and with Clear Text login enabled on a Public IP, you should set the clear text login to no and "Enable SSL for POP3 service".
You shouldn't need the "Webmail MTA hostname" set at all in either the Global Settings or the Server settings, if they are enabled then the server setting will override the Global. You should also enable "TLS Authentication only" to ensure your external clients are secure.
Make those changes and try connecting again using the
correct ports for POP3 or IMAP.
I would strongly recommend that you use IMAP instead of POP3 for client connections, they'll have a much better experience with that.