To log email traffic for troubleshooting in Outlook:

• Select Tools | Options... from the menu in Outlook.
• Go to the Other tab.
• Click Advanced Options.
• Make sure Enable mail logging (troubleshooting) is enabled.
• Click OK.
• Click OK again.
• Exit and restart Outlook.

Now it's time to reproduce the error by attempting the failed email action

[NOTE: Don't forget to disable "Enable mail Logging" once you've created the log files] 

Find the Log Files

While turning transaction logging on or off is easy in Outlook, finding the log files can be a bit more involved:

• Select Run... from the Start menu.
• Type "%temp%" (without the quotation marks).
• Click OK.

• POP and SMTP:
  • Open the OPMLog.log file in the folder that opens.
• IMAP:
  • Open the Outlook Logging folder.
  • Go to the folder named after your IMAP account.
  • Open the imap0.log, imap1.log etc. files.
• Hotmail:
  • Open the Outlook Logging and then the Hotmail folder.
  • Open the http0.log, http1.log etc. files.