That depends on how you obtained the customer information:
Contacts who signed up using a subscription form on my website. Yes. If you have a subscription form on your website, which clearly states “fill out this form to sign up to our email newsletter,” and are aware that they are signing up to an email list, this is definitely an opt-in list.
Customers who checked a box saying “please add me to your email list” when submitting form on website. Yes. Any customers of yours who submit a form on your website are considered opted-in. However, it must be explicitly stated that they are agreeing to receive a newsletter and/or email marketing campaigns from you. It is also best to send a follow-up confirmation email to ensure they qualify as “double opt-in” when using this method, to be absolutely sure they did not check that box by accident.