Without getting too technical, like other email read receipts across the internet industry, RSVPify relies on a tracking pixel to detect whether an email invitation was opened by a given recipient.

A tracking pixel is a tiny, transparent image placed within the body of a sent email. Although the pixel isn't visible to the email's recipient or sender, when an email is loaded, the transparent image is also loaded -- signaling to RSVPify that the email was opened.

Some email clients (notably Microsoft Outlook), however, do not load images without a proactive action required by the recipient. 

If the recipient does not choose to load images on your email invitation, there is no way for RSVPify to determine that the email was opened.

The recipient would still be able to see any text-based components of your invitation and links, but any included images (including the tracking pixel), would not have been loaded or seen by the recipient.

In recent years, email providers have made changes that are making email open rates less reliable. Many non-human actions trigger an open event, such as anti-spam filters, firewalls, automatic image loading, and autoresponders.