port 25 blocked

Dear all,

We are running on Exchange 2000 server.We have around 50 POP3 users.The problem is, some of the ISP has blocked the port 25 instead they have opened the port 587, our POP3 users are not able to send the mails.

1. How to resolve the issue ?
2. As an administrator, whether i have to do some configuration changes on our exchange 2000 server.

Regards,
Johnson