First of all, backup any and all email communication you have had with the company about it.
If neither of these are true, and you are in fact marketing to customers but not using their list or software, gather logs and customer testimonials, and proof about where you got the customer contacts.
Additionally, gather emails indicating why you left the company or were asked to leave, as it might indicate the company's intent to get even with you.
Lastly, talk to a lawyer. Consultation is often free for the first time.
If neither of these are true, and you are in fact marketing to customers but not using their list or software, gather logs and customer testimonials, and proof about where you got the customer contacts.
Additionally, gather emails indicating why you left the company or were asked to leave, as it might indicate the company's intent to get even with you.
Lastly, talk to a lawyer. Consultation is often free for the first time.