Webmail is Failing to Send

All of a sudden my webmail is no longer sending. It reads in part: This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

My mailbox is not full. It has done this to every person I have tried to email for the last hour. It was not a problem earlier today. It can't be on their end because the emails I am trying to send are to all different types of accounts. Has anyone else had problems like this? Any idea how I can fix it? Thanks!