In 99.9% of cases a problem is on your mail server side. In 99.9% of cases your mail server administrator uses outdated and/or unreliable spam protection technologies. Our specialists have a good experience in the IT, over the years we have met, unfortunately, stubborn administrators who use 'old-school' methods of fighting spam. Among the unreliable methods are methods that evaluate the reputation of an IP based on some kind of invented algorithms. Such serivises ALWAYS give a false alarm. If your admin uses them be sure that legitimate correspondence will be filtred too. As an example of such UNRELIABLE services is www.ipqualityscore.com, where they believe they use smart algorithms, while they give 40% of false decisions on clean IPs with good reputation as ours.
Administrators of private companies or corporations often have that problem of incorrectly configuring mail servers (using unreliable anti-spam technologies). An example of this is AT&T servers. If you have a mailbox on AT&T servers your ligit correspondence will sometimes be filtered and we can do nothing about it, because AT&T admins do not even consider themselves obligated to respond to complaints - as they directly state on their website.
Our servers have a 100% positive reputation, they don't send spam (it's technically impossible in our configuration), they are not proxy and no one can use them uncontrollable. Therefore, any attempts to block them is the ignorance of others admins.
The only solution for you to this problem is to use a different email address.