A single user is not receiving notification emails for new tasks assigned to them. It is not in their spam. They do receive emails from the Send E-Mail Smart Service though.
This has been tested against multiple other users with the same email notification settings group assignments and all of them received the task emails.
Outbound emails is also enabled in the Appian console.
Is there logs tracking the outbound emails? I couldn't find it in the Appian Docs - System Logs page.
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To troubleshoot, create a new dummy user account (but with the actual email address of the user in question) and assign that new account a task, and see if the email comes through.