Hi ​@specificmoon! Thanks for asking your question to the Community. I have a few ideas for you. 

1. Have you tried using an address validation tool to identify and suppress risky addresses from your subscriber list? There are several tools available, such as ZeroBounce. This would enable you to remove any spam traps, invalid addresses, or other risky profiles from your audience. 
2. Have you verified that your sender infrastructure meets the requirements for domain & IP authentication? There are specific requirements for your sender domain DNS records to ensure that your emails are properly authenticated and delivered by the mailbox providers. If these are not in place, Hotmail/Outlook might be filtering your emails to spam or not delivering them at all. More info here: Understanding email authentication
3. Have you examined your Hotmail/Outlook subscribers to determine if the “opened 5+ emails recently” are machine or human opens? You can create a segment similar to the example below to isolate profiles where Apple Privacy Open equals true. This could give you some intel. 

   

    

I hope this is helpful. Let us know how it goes, and if we can help further. 

thanks for taking the time to respond 

all good regarding the above, no red flags. 

we’ve been using Klaviyo for years, no issues like this before, bar the odd spam warning which are typically sorted quickly through the techniques above,  hence me flapping.

the latest email to the super warm HMOL audience hasn’t even reached 10% open rate.