Hello @LM0721, welcome to the community!

As a general advice, you should refrain from putting in marketing messaging in “transactional messages” because then it wouldn’t be a transactional message - it would be marketing related.  There are some loose interpretations of that, and you *might* get Klaviyo to approve it, but it’s a dangerous road to go down because it’ll work until one day it doesn’t, and then it’ll be tough to start over.

It’s hard to pin point the exact definition of a transactional message (it’s all just email, right?), but to me, a transactional email is a utility message to complete or fulfill transactions (hence the term).  I’m not a lawyer, so there are definitely better legal interpretations, especially in certain regions (e.g. GDPR in the EU), so you should consult with counsel for regional rules and legal implications. 

But from a practical standpoint, anytime you send emails for those that did not opt in, risks the ill-fated “Mark this as Spam” complaint in the recipients’ respective inbox providers.  These are very strong signals (as opposed to simply ignoring or not engaging with an email), and can impact deliverability despite your high open rate.  In fact, it’s probably because you only send emails to those who expect it (and want it) is why you have a great engagement score to begin with - don’t ruin it!

For those who haven’t engaged or opted-in, use retargeting popups on your site targeted to those who are in that Segment of not opted-in or Sign Up Form Teasers.  Find other value exchanges or incentives for people to opt-in (if they haven’t already).  Earn their trust, and their permission and you will not only build a more valuable List, but also sleep better at night!

Hope this helps.