The following users do not have email address specified

 The following users do not have email address specified 

I faced the issue with one of My UAT user, says that get the below error message while configuring alerts. 

Error: The following users do not have email address specified

     Problem Description: Unable to set alert on a List for single or multiple users 

    cause: below are the possible causes for the issue.

·         User don’t have the mail box assigned in AD

·         Users profile is imported to SharePoint before he got assigned to mailbox.

·         The user’s profiles are not updated in SharePoint even though everything is setup up correctly in Active Directory and in Exchange