How does the open / read tracking in MailMerge365 work?
If a campaign has open tracking enable an invisible image is added to the bottom of the email. When the email is opened by the recipient this image will be loaded and MailMerge365 registers this event.
Some email clients (e.g. Microsoft Outlook) will not load images by default, which will result in MailMerge365 not registering that the email was read (until the recipient clicks an email or activates the loading of images).
Gmail uses an anonymization proxy for images which does not affect MailMerge365, because all emails are sent out individually with individual links.