Hi @Konic Group
Just to clarify @stam_marko comments:
You can technically send campaign and flow emails to any contact whether they have subscribed or not (never subscribed). However, Klaviyo will not allow you to email any contact that has been suppressed. Reasons for suppression include unsubscribes/flagged as spam, invalid email, bounced, or you've manually suppressed them.
Should you send marketing emails to contacts who have never subscribed? In general, the answer is no for ethical/reputation reasons as they haven't opted-in. Engagement will likely be poor anyway and risk of unsubs/spam flagging. In any case, in the the UK/Europe (where I am), GDPR regs are stricter than in other countries: contacts must provide explicit opt-in to receive marketing communications. In other countries, such as the US, the CAN-SPAM Act only requires that a contact has the ability to opt out of receiving marketing emails so that may be a factor for you.
On the point of transactional v marketing emails, I think there are some grey areas! Are abandoned checkout emails "transactional"? Strictly speaking, they are not because they're not related to an order that's been placed. However, they've explicitly started a purchase but not yet completed it. I would argue that these types of emails, if showing details of their incomplete transaction and you're helping them achieve that goal, can be sent to 'never subscribed'. However, I only send browse/view abandon emails to opt-ins as there is no explicit intent yet to purchase. Others on here may have different views!
Hope that helps
Andy