Hi @ADeStefano, I see this is your first post, so welcome to the community!

I’m assuming you’re on one of Klaviyo’s supported ecommerce platforms (Shopify, BigCommerce, etc) - on all these platforms, Klaviyo syncs all the platform’s emails (sometimes called customers in your platform, even though they may not have purchased yet) to Klaviyo so they can start tracking other behaviors. 

Ecommerce Platform

How your platform gets email addresses (separately from Klaviyo Signup Forms) are typically through your website registration (creating an account), when they checkout but don’t complete the order but opt in for email, or when they make a purchase.  These all come into Klaviyo as a “profile” - some may go into a specified List if you have that configured in your Integration settings - it looks like this:

3rd Party Applications or Via API

Another way you may be getting email addresses is through 3rd party plugins or apps you may be running that captures email addresses.  Along the various tools like 3rd party popup tools, quiz or survey tools, helpdesk or onsite chat programs (and possibly others!) might be all adding email addresses (profiles) to your Klaviyo account.  

Manually Uploaded

Lastly, the most obvious reason why you have email addresses in your Klaviyo account might because you uploaded them, or migrated from another email platform at point, or maybe someone on your team uploaded a List and the List was later deleted (deleting a List doesn’t delete the Email Profiles necessarily). 

Marketing Consent

Now the ultimate question - can you email everyone?  The short answer is probably not a good idea, but you might be able to if you can follow each of your email sources above and make sure you have requested proper marketing consent.  Most tools have explicit messaging or labelling so that the subscriber understands they are opting in for marketing communication from your brand.  In some regions, this is legally required - but for all cases, sending emails to people who weren’t expecting it or not wanting it is just not effective and affect your email deliverability caught into Spam folders. Klaviyo has a great explanation guide on this here:

• Understanding explicit vs. implicit consent

Hope this helps!