BigBusiness 15 lets you drag to the Docs folder including from Windows Thunderbird.
Drag email from Windows Thunderbird and drop on the Docs folder to save time. Other users can open the email by double-clicking and see any attachments included.
Turn on Docs if you haven't already and drag a message from Windows Thunderbird. Drop it on the Docs folder on a new order to save a whole email including attached PO.