There’s several reasons why an isp may block your server ip. This is extremely common and happens to most servers, especially large free isp’s like Gmail, Yahoo, Hotmail, etc. because they are so sensitive. If your ip is blocked, below are the possible reasons why it may have been blocked.

1) Spam from the server
This is the most common reason. If you get any spam complaints from your datacenter or receive any bouncebacks that you did not send, these can be used to trace the source of the spam. If you do not have any spam complaints or bounces to trace, then it is harder to find any spamming activity but the mail queue and mail logs can still be reviewed for unusual activity.

2) Incorrect hostname/IP settings
For an isp to accept your mail, your hostname must resolve to the main ip and the Reverse DNS record (also known as PTR or RDNS record) must match the hostname too. To check this, you can use the simple nslookup command and type: nslookup my.hostname.com and then that should resolve to your server’s ip. Then do the same for the server’s ip: nslookup 1.2.3.4 and then that should match the server’s hostname. If either don’t resolve or don’t match, then most isp’s will not accept your mail.

3) Mail authentication not setup
Many isp’s now are requiring mail authentication tags to be added to the DNS zone for verification to prevent spoofing. To increase your mail acceptance rate, you can add an SPF record and DomainKey record in cpanel. You can login to CPanel for the domain you want to do this on, go to the Mail section, click Mail Authentication, and you can add the SPF Record and DomainKey Record there.

4) High volume mail activity
Sometimes even if there is no spam being sent from the server and all your settings are correct, many isp’s will block you if you send too much mail to them. Many large free isp’s limit how much mail you can send to them per day, and if you exceed this limit, they will block you either temporarily or sometimes even permanently.

5) Contact them
If there is no spamming, all the settings are fine, and the mail volume is normal, then the only way to get unblocked is to contact the isp. The contact has to come from the owner of the server (you) or the owner of the ip (your datacenter). In the headers of the blocked email, there is usually an link showing how to contact the isp to request delisting.