Obtained outside email list

Hi! I would send a campaign just to this list (add a spot inviting them to sign up for SMS too!). Use this performance to inform your normal campaign strategy. If they are all very engaged, I wouldn’t worry about re-warming too much! If they aren’t as engaged, those that did engage will be pulled into your Active 1 month segment! 

Welcome to the community ​@kimmichiganbythebottle 

No, do not send to those 800 contacts in a single campaign!

If that list is contacts who registered for the event but did not explicitly opt-in to received emails from your business then such a list would not comply with Klaviyo's acceptable use https://www.klaviyo.com/legal/acceptable-use-policy. That clause here:

"You are prohibited from sending to: Email addresses, phone numbers, or other contacts that are purchased, rented, borrowed, third-party owned/provided or any other lists that have not consented to receive communications, promotions or advertisements from your business".

The grey area: if, when those contacts registered for the event, they explicitly opted-in to receive emails from event sponsors or 3rd parties. Check with the event organiser. If so, I think you may be OK (but they are unlikely to remember having done that!).

However, if they've never engaged with your business apart from maybe seeing your sponsorship logo, presentation, or walked past your booth at the event, engagement with a campaign will be low and hitting 800 contacts will likely negatively affect your metrics (opens, opt-outs, spam complaints) and your sender reputation.

If they have explicitly opted-in to receive emails from event sponsors, and you still want to send a campaign, you could treat those contacts like a warming re-engagement campaign. 

Without knowing your business, I can't make specific recommendations apart from you offering something relevant to that audience and of value (case study, guides etc) and splitting the list into a much smaller campaign segments, say 100 contacts, and test the response before moving on to the next 100 if the results are positive.

I would also ensure the subject line and content make it clear your association with the event to maximise engagement. Finally, I would include a 'not interested' button in your email that captures that click in a suppression segment so you can identify and suppress them - that approach should minimise some hard unsubscribes.

Not what you wanted to hear, but I hope that helps!

Andy