Sorry for the delay
The key thing you need to understand before you embark on this is that your current employer owns ALL the work product you have generated for the newsletter you are creating.
The law is clear; absent an agreement to the contrary, intellectual property created by an employee
for an employer belongs to the employer.
So any templates you may have created, or software tools or images you may have created can NOT be used in this new business. Period. If you do, you are opening yourself up to a lawsuit by your current employer.
1. Say your have a template or software program for your current newsletter and you modify it for one that you are going to sell to another company? In that case, you would be liable to your current employer for using their property to sell to others.
2. Say you created a image for your current newsletter and one of your potential customers wants to incorporate that into their product. Again, that would be in improper use of the current employers property.
So long as you are creating the newsletters for your new customers from scratch, and not using ANY tools, images, programs created while working for your current employer? You will be fine.
Let me know if you have more questions, happy to help if I can