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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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!