After Confluence runs for a while (hours to days), users notice that none of the email notifications are being sent. When I check, the Mail Queue is empty and the Error Queue is full. I see the following error repeated in the stdout.log many times.

2005-06-15 10:46:01,942 ERROR [atlassian.core.task.ErrorQueuedTaskQueue] com.atlassian.mail.MailException

The log messages do not include a stack trace. I cannot find any other log information regarding this error. When I "Resend error queue" the messages move to the Mail Queue and shortly thereafter they are put back into the Error Queue. When I restart the Confluence service, the Mail Queue successfully flushes itself automatically, but the problem reappears after a while.