In Outlook Express I’m getting the error message. What should I do?

The full error message in Outlook Express is:
• Some errors occurred while processing the requested tasks. Please review the list of errors below for more details.
• Some of the messages in your Outbox Folder could not be copied to your Sent Items Folder. Your server has unexpectedly terminated the connection. Possible causes for this include server problems, network problems, or a long period of inactivity.