However, this is not the case here and it is not displaying the yellow exclamation mark or any error message. Regardless, I have checked the most widely recognized reason, that the email is so large due to attachment and graphics. This is something I have never seen before and suspect might be unique for the Mac Outlook. After that, I am unable to send any other email message even, I am not receiving new emails. “ Help! I am using Mac Outlook 2016 and an outgoing message is stuck in its Outbox.