I just tested this from one of my emails to another and it worked fine. I suggest that you log into your account at your providers web mail page and see if the message is going to the Spam folder. Otherwise contact Craigslist and see why it does not work for you. Thunderbird has no interaction with this send process and can only download messages that actually arrive in your Inbox. If the message does not arrive in your Inbox then Thunderbird cannot download it.