It seems that the python-ideas mail server really doesn't like my attachment, so it's time for Plan C. You can download the code from here: http://www.cosc.canterbury.ac.nz/greg.ewing/python/yield-from/Threads.zip Is the mail server configured not to accept attachments or something? The error message I'm getting is Your message cannot be delivered to the following recipients: Recipient address: python-ideas@python.org Reason: SMTP transmission failure has occurred Diagnostic code: smtp;554 permanent error Remote system: dns;mail.python.org (TCP|132.181.2.71|1204|194.109.207.14|25) (bag.python.org ESMTP ) -- Greg