Will marketing automation work on messages that have been sent already but are still in delivery?

Yes. You may select these types of messages for the condition of message opened or clicked. Both conditions only affect those contacts that have not interacted with the message at the time of turning on the workflow. If you use Perfect timing, Time travel or A/B test to send a message and contacts open it before you publish the workflow, we can’t track these actions. We track all interactions with your message after you publish the workflow.