Emails being sent from our webserver are reported as spam

We have several websites on which visitors can sign up for various things, such as newsletters, warranty registration, etc. The sites use a php sendmail script to send out those emails/confirmation emails. The issue we are having is that for some reason, our webserver IP address is being reported as a spammer with CBL. How can I prevent this from happening?

Additionally, we have two mail domains. Our main one is, however we have recently started using as well. Email sent to a user at either address will go to the same mailbox. We are properly setup with Reverse DNS, etc. with the domain, but not with the one. How do I go about setting up the with proper reverse DNS, etc?

The mail being sent from the webserver is using a address, so I don't know if that is a contributing factor to the issue. Your help is appreciated - thanks.
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You don't need to set up "reverse dns" for domains, only for IP addresses, so I don't think you need any new "reverse DNS" entries when adding a domain to your e-mail server.

If you're being blacklisted because your web server is allowing anyone to "sign up" and receive an e-mail confirmation, there's not much that you can do. Perhaps someone has used your forms to subscribe people who didn't want the e-mail. You may make it harder to fill out the forms by automated means by making it multiple pages or using a CAPTCHA.

If you're being blacklisted because your form processor is able to be manipulated to send spam to anyone, or if your web server is otherwise compromised or running other vulnerable scripts that can be used to send spam, that's another story. You should look at the logs of those visiting your web forms to see if you can find anything suspicious, and also start logging outgoing traffic on port 25 from your web server to determine if you have a security problem.
yes not having reverse DNS is a prime cause of a suspect SPAM, you would need a reverse PTR record in DNS for your Outgoing Mail servers name pointing to its IP, if your ISP hosts DNS for you then you will need to make a request at your ISP for this,

so if you do nslookup with your Outgoing Mail servers name ( FQDN) then it should point you to the correct IP
welshivAuthor Commented:
The outgoing mail server is setup correctly - the mail in question is being sent from the webserver, not the mail server
Don't use PHP's mail() function, it's horrible. Get the pear mail package:

It enables you to set the delivery method to SMTP rather than using the local server's sendmail. Set it to relay to your actual mail server so your mail server is tasked with delivering the messages.
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