You can trigger an autoresponder when someone opens an email campaign that was sent out previously. For example, say you sent out an email campaign offering customers a limited-time-only special discount. You might want to send out a follow-up campaign to the customers that opened that email a few days later to remind them. You could use the "open" trigger to accomplish this.
If you use the open trigger, you will need to select which campaigns Direct Mail should monitor for opens. Campaigns that have already been sent out aren't eligible, but campaigns that are scheduled to send out in the future, or draft campaigns are eligible.
If a recipient unsubscribes or is removed from your mailing list before they open the triggering email campaign, they will not be sent the autoresponder.