Contact your domain registrar
The most common problem we see is that the “PTR record” needs to be established. PTR records resolve IP addresses into hostnames. The explanation is here. Basically, the ISPs 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 online. Some of it is even written in English.
Contact your domain registrar and tell them you want a PTR record. If you have an IT administrator or communications IT specialist, they probably know how to do this too.
That should take care of most of the problems.
Don’t forget the obvious. Have your customers check their spam folders.
Contact your Internet Service Provider (ISP)
For anything else, you need to get hold of the ISP and find out exactly why they are rejecting your email. You may also need to let them know which IP addresses will be sending emails for your domain.
The IP address that Webtix uses *could* change. However, it should not. The only time the IP address would change is if you changed your server configuration and went to a different email setup.