We have a couple of user who are not receiving the task email, but the rest of our user have not problems. We are using Appian 17.1
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Have you checked in spam? Sometimes the mail is there in spam folder.
In addition to checking spam
(1) make sure their email address is entered correctly
(2) check their notification settings https://docs.appian.com/suite/help/19.4/Notifications.html
(3) Is there any difference in those users? meaning are they external and the ones receiving are internal? If so, check SMTP policies
Hope that helps!