You have a drip campaign finalized and you realize you need to add more contacts to it. The answer to this question depends on the mailing type of the kickoff mailing for the campaign and if the campaign has already been started.
1. Kickoff mailing is a regular mailing, the mailing HAS NOT been sent yet, and the campaign HAS NOT been started-
As long as you have not started the campaign or sent the regular mailing that will kickoff the campaign, you can add contacts to the segment or category assigned to the mailing.
2. Kickoff mailing was a regular mailing, the mailing HAS been sent, and the drip campaign HAS started-
You cannot add contacts to a campaign that kicked off from a regular mailing that has already been sent.
3. Kickoff mailing is/was a triggered mailing-
You can add contacts to the triggered segment anytime and they will be picked up in the campaign the next time the triggered mailing runs.