When a user sends an email using an email alias, email opens are not tracked as expected in Salesforce. This issue occurs when both Salesforce Inbox and Einstein Activity Capture (EAC) are enabled, but email capture is disabled.
This article explains why email opens aren't tracked when sending from an email alias with Salesforce Inbox, and what to use instead.
Use the primary email address instead of an alias when sending emails so that the email gets tracked.
This behavior is observed specifically when using email aliases with Salesforce Inbox and Einstein Activity Capture (EAC).
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