Canary Mail offers read-receipts that are implemented using an invisible image. Unlike the traditional implementation of read-receipts as described in the IMAP specification, which many modern email providers and apps no longer implement or respect, the modern image-based implementation works reliably and is therefore used by popular email marketing software and apps.
Canary Mail's read-receipt implementation prioritizes privacy - Canary only shows when and if an email is read. Canary does not collect or present any associated metadata such as IP addresses, location or device info, how many times an email is opened, etc.
In order to sync read-receipts across all your devices, Canary uses a minimal server footprint that only leverages anonymized unidirectional hashes. No personal data or email content is sent to server for this purpose.
Canary Mail does not offer per-recipient read-receipts since that involves a serious privacy issue - in order to implement per-recipient read-receipts, emails must be routed through a 3rd party SMTP server instead of the user's email provider. We think this is unacceptable from a security / privacy point of view.
Click here for more information on Canary's privacy features & policies: https://canarymail.io/privacy.html