Comparing a path an email takes to reach its destination vs. driving a car on a road trip, emails do not travel directly from point A to point B. The final destination is more like point W, and the email is bouncing from one server to another at the speed of light while being filtered at each step. If your emails are denied by any spam filter along this journey, it simply is not delivered.

It’s not anything the sender can control. It is the receiving system that determines what is spam and what is not, and that is completely up to the organization that sets up the filtering. Spam filtering is updated multiple times daily, and it is beyond any one person’s capacity to know what is placed in the updates of the many spam filtering systems worldwide.

You may also have ended up on spam blacklists for some reason, which you will need to ask to be removed from.

You can check to see if you are on common blacklists using these tools: