[imp] IMP configuration for webmail from internet

[Eelko]

In a local network I can use IMP for reading my mail on my mail server.
Now this LAN is also connected to the internet by a router. This router
blocks all incoming traffic except SMTP and HTTP.
Unfortunately I  am not able to access my webmail service from the internet.
Do I have to allow more ports?
Or do I have to set a special configuration option on the mail server?

All comments are welcome.

Eelko.