If you are trying to receive mail from Internet, you have 3 ways:
1. Set up POP3 Proxy in WinGate, and make your clients use it. You should use delimiter like '#' in email, for example, clients email box on outside Internet mail server is
'brandon@mail.com'. Your WinGate computer name is 'gate'. So, on client machine you should set up POP3 server to 'gate' and user name to 'brandon#mail.com'
2. Set up transparent redirect in NAT. So, when somebody is trying to access outside mail server, request will be passed to internal POP3 proxy.
3. You can just open port 25 (SMTP) and port 110 (POP3) in WinGate NAT for everyone. Do it in 2 filters, because filters are used in OR like, while conditions within filter are ANDed. So, you can't just put 2 ports into one filter rule, because filter will work only when user is trying to connect on both ports - 25 AND 110.