Emails scheduled for delivery never send

Mail delivery threshold is set to default value of 5. It seems that any time there are more than 5 recipients, the email status shows, "New-Ticket E-Mail scheduled for delivery to..."

But then it never emails any of the recipients. There is nothing in the outgoing email history section when this happens; no errors, no successes. There are successful connections logged during the timeframe we would expect the message to send. No timeouts to the mail server. Email send works perfectly fine otherwise.

Any suggestions appreciated.