How to delete sent mail which failed due to outdated email address – not visible in “Sent Mail”

I sent an email to a group last night.  One of the email addresses was incorrect.  I keep getting error messages that it won't send but I can't find it in "Sent Mail" to delete it.  I've searched all sent mail and still can't find it.  Any ideas on how to find it and stop the annoying message from popping up every time I send/receive?

Thank you.