What constitutes an email list?

I have a SaaS product that people have created an account to access - which they've used their email for. Are these email addresses considered part of my "email list", or do I need to collect the email addresses in a different way for me to be able to send emails to the addresses with new product information (which is unrelated to the SaaS that they created an account for, aside from the fact that I'm building it).