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Once the recipient list has been constructed from all the sources, you can review the final list of users to determine any recipient/s who should not be on the list of receiving the mail. This is ideal in cases when selecting Custom Fields and/or Entities.
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From this screenview, you should have the ability to be able to Exclude or Include recipients from/to the notification.
The source column is the way to determine where the email address was obtained from - was it an Entity, CustomField or Manual insertion? The Key reason behind this is mainly finding out the source of the email and ability of being column is to provide the ability to be able to control the order of the view as per the list based on the source.
Useful information:
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