What: This is an exclusive Mailtrack feature that lets you know about people using non-trackable email services (around 5%-10% of recipients are non-trackable).
How: When you hover over the checkmarks in Gmail it will say 'Warning: non-trackable recipient detected...'
Why: Mailtrack has detected in the past that the recipient of your email uses a non-trackable email service. Some recipients use email services that are not trackable by Mailtrack or any other email tracking service. We feel that it is preferable to warn you of this, instead of providing inaccurate information.
Long explanation: Email tracking works by adding an invisible image (pixel) to each email you send. When your email is opened by the recipient we detect that the image has been opened. However, some email services (e.g. Outlook) don't always allow the images inside emails to be opened, meaning email tracking is impossible. This limitation not only affects Mailtrack but all other email tracking services that use images (pixels) to identify email opens.