Thank you for the additional information.
I'm very sorry, but by contacting the clients, you violated the contract with your former employer. Not only that, because you did so for your new employer, if you do not cease doing so as demanded in the form letter---and it is a form letter because their attorney specifically drafted it for them to send out before they incur legal fees fin suing their former employees-----they will in all likelihood sue both you and your new employer.
I'm afraid that your employer is not legally savvy and they should be concerned about the letter, as should you.
While not relevant, it is important to understand that the term right to work state refers to the fact that you cannot be forced to join a union in order to work---nothing more. It has nothing to do with whether it is legal to appropriate customer lists from one employer and use them for the benefit of a later employer.
Now, CA does not allow is a non-compete agreement which would prevent you from going to work for a competitor. So you cannot be told that you cannot go to work for a competitor in CA---like you can in virtually all other states. However, your former employer's client list, contact numbers and contracts with the customers are considered the property of your former employer and you can be sued for interfering with the business relationship they have with their customers, and for theft of trade secrets based on your use of the information you have acquired while working for the former employer.
If you want to avoid being sued, then I would strongly urge you to cease all contacts with the customers of your former employer, and sign the document stating that you will cease doing so. If you are sued, you can bet that your present employer will be sued right along with you. You will likely be terminated as your employer's attorney will suggest that they must do that to show good faith and you will probably end up having to defend yourself against the lawsuit. Further, you won't be eligible for unemployment benefits in this situation.
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