You can trigger an autoresponder to send when someone opens an email campaign. For example, you may send out an email campaign offering customers a limited-time-only special discount. You might want to create an autoresponder that sends out a follow-up "reminder" campaign to any customer that opened the original email. You could use the "open" trigger to accomplish this.
If you use the open trigger, you will be asked to select which campaigns Direct Mail should monitor for opens. Email campaigns that have already finished sending out aren't eligible, but campaigns that you have scheduled to send out in the future, or draft campaigns are eligible. If you have created a message that you would like to monitor for opens, but you have not yet scheduled that message to send (or saved it as a campaign draft), you will need to do that first before it appears in the list of eligible campaigns.
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.
Open triggers and Mail Privacy Protection
Apple’s Mail Privacy Protection (MPP) feature can cause autoresponders that use an open trigger to send their message even if the recipient has not actually opened the email. If a recipient is using MPP, their device will automatically "open" all emails in the background as soon as they are received, even if the recipient never actually views the email themselves.