How can I get POPcon to not work with my ISA-Server (proxy) ?

You need to open the IP ports for POP3 and SMTP in your ISA Server. Create two packet filters:

One IP packet filter that allows POP3 accesses with the following settings:

• IP protocoll: TCP
• Direction: Both
• Local port: dynamic
• remote port: fixed, number: 110
• local computer: standard external ip address of the ISA computer
• remote computer: all remote
  

One IP packet filter that allows SMTP accesses with the following settings:

• IP protocoll: TCP
• Direction: Both
• Local port: dynamic
• remote port: fixed, number: 25
• local computer: standard external ip address of the ISA computer
• remote computer: all remote