Hi Krisi,
first of all, I want to introduce myself. My name is Ismail Keles. I am a computer science student from Augsburg, Germany, who has a part time Job in IT Administration.
I was looking for new projects here, so I saw your description of your problem. In our company, we had the same problem, that our employees could not send emails to specific customers. That happened after we changed our domain.
If I understand your problem correctly, your emails are landing in the spam boxes of your customers and you want to avoid this.
I hope I can help you by taking the same measures we did in our department. If not, you do not have to pay me. I know that this is a very big problem because we had the same problems with our customers who were angry because of not getting informed whether their order is in process or not.
I am looking forward to help you out of this situation.
Best regards,
Ismail