please check whether they are providing a pop3s service.
When not you should try to use imaps-service.
Additionally check if your credentials are correct.
Please check with IS Administrators Guide how to configure an email port, might be that you have to specify some additional properties to the java command line in setenv.sh or custom_wrapper.conf or both.
with gmail. check if you are using application specific password. You need to generate a password for Integration Server from gmail account than using your actual username password.
Were you able to fix this issue? I’m also facing this and wondering if you could provide some info. I’ve tried using it in my company to connect to official email address but getting same error, then I tried connecting to gmail on my local pc but same. Any help will be appreciated.