Each time you email campaigns via your platform, the system embeds a tiny invisible image pixel at the bottom of your HTML email.
This open tracker image pixel is unique to each campaign and for each contact.
When someone opens your email with images turned on, that pixel is downloaded from the mailing server, and it's recorded as an open on your campaign report.
Automated responses, i.e. out-of-office messages, generally do not download this graphic, so they shouldn't count as opens.
Open tracking via the image pixel is the standard process and is a great tool to give you a general overview of your contact's engagement, but it does have some limitations.
Open tracking depends on this tiny image pixel hence it only works in the HTML email and not in the Plain-Text campaigns, since you can not include the images in the Plain-text email campaigns.
Similarly, open tracking won't work if your contacts or their email client have chosen not to display images.
In most of the email clients, the images are by default disabled hence system won't able to track it.
E.g. In the Outlook Email Client
Most of the web-based email inbox provides images visibility are by default enabled but it won't work in the spam or junk folder.
You can ask your contacts/subscribers to add you (sender email address) to their email address book or enable the image's display option in their email client to get more accurate open tracking.