A few Practifi clients have reported issues with the Salesforce Outlook add-in. Many of these users are using the latest Mac OS version -- Big Sur 11x. This is a known Salesforce issue documented here, but we felt it should be documented in our knowledge base as well.
Currently, the Salesforce Outlook web add-in is not supported on Mac OS Big Sur 11x with Outlook for Mac versions 16.44+. In short, this is due to new functionality that Apple has introduced to prevent cross-site trackers and cookies. Per Salesforce, they are working with Apple and Microsoft to come to a resolution, and progress toward a fix may be tracked here.
If affected, you can use the Outlook Web App on Chrome or Safari. If in Safari, be sure to deselect "Prevent cross-site tracking" in Preferences.