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I don't recall seeing this in the last version of Sendy, and based on my open rates from old emails , I believe this to be a recent change.
It appears that all email sent to Gmail addresses automatically show as Opened. While it's fun getting excited about open rates, it just doesn't make for accurate data. Is this a change in the way Sendy handles Gmail or is it a change on the Gmail side?
It's not something to do with Sendy but Gmail.
When a user opens your email, a 1x1 pixel invisible image hosted on your Sendy installation is downloaded, which then translates to an 'open' (this is the same for any email marketing software/service).
In January 2014, Gmail began automatically downloading images for emails viewed in a web browser or Gmail mobile apps, resulting in opens to be tracked automatically. If the Gmail user checks his/her email in a third party mail app like Outlook, Apple Mail, the tracking image will only be downloaded when he or she opens the email.
Also note that even if someone opens your email in other third party email clients, open tracking also depends on whether the user's email client has 'images turned on' when viewing your email. Some users prefer images to be turned off so that 'opens' cannot be tracked.
This doesn't just affect Sendy or yourself, but all email marketers in general. Open tracking needs to be taken with a pinch of salt.