The common reasons are:

 

1. Sender sends bulk emails, such as newsletters, invitations, or e-card, etc.

 

2. Email account is hacked to send mass email. Most of these cases happen due to the simple password of email account, such as 123456 or qwerty, etc

 

3. Trojans Horse virus or spywares on sender's computer to send out spam email.

 

 

 

You may say that you are not the spammer, but your SMTP's IP is still in the RBL. It is because that your SMTP's IP, provided by your ISP or ESP, is shared with other users.

If the spammers and you use the same IP, you are the victim.

Before the IP being released, you can send out emails by other SMTP's IP or ask your email service provider to give you another clean IP.