When an email is stuck at PROCESSING status for a long time, check the Sent count first, 0 means that there is no email in the recipient list, so email cannot be sent out and stays at PROCESSING status.
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If recipient list is a dynamic list, please refer to View/Edit Saved Dynamic Recipient List to ensure the Registered for settings are not conflict with Not registered for settings.  
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  • Note: To verify if recipient list has email address, select it to your email, the system will display how may recipients are included
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If Sent column shows a non-zero number and email is still stuck at PROCESSING status, please Contact Support