What happens if I add more messages to my Campaigns at a later point?
Campaigns are open-ended! This means that subscribers will continue through to receive your Campaign messages as new actions are added to the end of it, regardless of whether the message was in the Campaign at the time the subscriber was added.
For example, if a subscriber has received two messages in a campaign, they will receive the next message you add to the campaign. This will happen no matter how long you wait before adding that next message to the campaign.
One thing to note is that this applies to all actions—including wait actions and tags. For example, if you added a new one-day wait action after the second message, the subscribers would enter that wait period and would be required to wait one day before receiving the third message.
If, however, you add a message or wait action between messages one and two, then the action would not affect subscribers who have passed that point in the campaign already and the message would not be sent to those subscribers. The subscriber would only be sent messages that are added after the messages the subscriber has already received.