Typically, this occurs if you previously set up delivery for an email message and canceled the delivery before it sent out.  This will cause the system to think that the email has already been sent out and use the built-in filter (recipients who have already been sent this email) to be applied.   To send the delivery, you will want to create a copy of the message and set up delivery for the copy.  Once you have successfully sent out the copy, you can archive the original message.