You'll need to check MX records for your DNS registrar. If you are using a dynamic IP, you'll need to sign up for a static or setup a Dynamic DNS host such as DynDNS or No-IP and then pay a small amount for the MX capability.
Also check that your ISP is not blocking port 25. If they are, most DDNS hosts can also do port forwarding so port 25 sends to port 2500 or something like that.
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