Often, Yahoo will put your mail server on a blacklist if spam is being sent from your public IP address.

Some of the key reasons that your IP may be blacklists are as follows:

1.) Compromised Web Script
2.) Compromised End-User Account
3.) Legitimate Bulk Emailing
4.) Forwarded Email

Please make sure you are doing the following:

1.) Make sure that your mail server is not an open relay. You can find these settings in the admin user interface of most mail servers. You should not be relaying mail for anyone.

2.) Make sure that you are requiring authentication to send out from your mail server.

3.) Scan your network for spambots and compromised accounts. Also have your users scan their computers for malware and viruses that may act as smtp engines to send out spam through their mail clients.

4.) Make sure you have a proper reverse DNS entry set up for your server.

5.) You may want to implement DKIM or DomainKeys. Yahoo likes when servers have this.

Once these things are complete you will need to request removal from Yahoo. You can fill out the Yahoo Mail Bulk Sender Form.

If you mail is still being blocked 48 hours after you submit the Bulk Sender Form, you will need to submit a Yahoo Mail Delivery Issues Form.

Before submitting the Yahoo Mail Delivery Issues Form, please be sure that you are abiding by Yahoo's Email Policies.

This should enable your server to be removed from their list even though it may take a few days.