In addition to addressing individual rejections I also subscribe to services such as emailreg.org which authenticates and verifies email senders and provides whitelists for email servers and ISPs. However, one of the things they want to know is which IP addresses will be sending emails for my domain.
The answer is our hosting provider did say that the IP address that Webtix uses *could* change. However, it should not. The only time the IP address would change is if we changed our server configuration and went to a different email setup. At this time, we have no plans to do this.
The most common problem we have had is the “PTR record” needs to be established. PTR records resolve IP addresses into hostnames. The explanation is here. Basically, the ISP’s check the return email address. That address has to come from a server where the reverse DNS lookup is the same as the “emailed from” server. There is a lot of other information on-line. Some of it is even written in English.
You will need to contact your domain registrar and tell them you want a PTR record. They will do it for you. It will take a couple hours to propagate.
That should take care of most of the problems. For anything else, you need to get hold of the ISP and find out exactly why they are rejecting your email.
BTW: Don’t forget the obvious. Have your customers check their spam folders.