My ISP(Sympatico) blocks port 25 inbound and outbound as well. If you want people to be able to send email with your server(not really needed as basically all ISPs have an SMTP server), you will have to set your server to listen to a port other than 25. Port 25 is the standard port for SMTP(sending email between servers, and sending email from an email client to a server).
Sounds like you want to be able to receive email on your server as well. This is not possible unless you set the server to receive on another port other than port 25. You can pay a company that offers to accept email for your domain name and forward it to you on another port besides 25. One I know of is dyndns.org, but there are many others. It's pretty cheap, especially if you have a lot of email coming in.
This service that you will pay for will be accepting your email for your domain. You tell the people providing the service the port your email server is listening on to accept email. They will then contact your server and send the email to you.
To send email from your server if your ISP also blocks port 25 outbound, you basically have to send via either your ISP's SMTP server, which will then sent the email to the proper domain, or you can use an outside server which is listening on another port.