Every machine needed an IP in order to connect to Internet and current Internet Service Provider (ISP) provide Internet Service with static IP and Dynamic IP.
First of all, you may look for your external IP by browse to the following URL.
Case 1: You are using dynamic IP.
If you do not purchase any fixed IP from your Internet Service Provider (ISP) such as Malaysia TM Streamyx then every time you connect to the server, you will be assigned with a dynamic IP. (Is like a temporary passport)
If the previous user of this IP address sent out Spam/virus mail or perform any illegal activity. The IP address will be listed in Spam database (like a police station record). You may be curious why some email recipient able to receive your mail but some are not even you are using the same IP that period. The reason is different email services provider refer to different Spam database. Example Hotmail that refer to "police station record in Malaysia" may block your sending or alert on your mail but "police station record in Singapore" that referred by Gmail will let your sending go through as they identify your IP is "clean".
You may regenerate new IP by restart your modem/router.
Case 2: You are using static IP.
If you have static IP but your IP get blocked by Spam database, check if your IP get blocked by which Spam database via URL of http://www.iptools.com, look for "Spam DB Lookup" section and type in your IP.
Spam database which show in red have blacklisted your IP from their database, click on the red highlighted Spam database name and look for the URL to request "IP removal".