There is an issue with Thunderbird 31.1.0 (released September 2nd) that has recently come to my attention. Mailing lists which contain a space in their name won’t work and Thunderbird will throw an error when attempting to send. This appears to be related to Bug 1060901 which is currently being worked. Depending on the severity of this bug, the patch may be deployed with the upcoming Thunderbird 31.2.0 release (scheduled for October 14th) or Mozilla may choose to do an emergency release which would be Thunderbird 31.1.1 once this bug is fixed.
In the mean time, the workaround is to use an underscore (_) in lieu of a space in your mailing list names. Example: “The Mafia List” should be renamed to “The_Mafia_List”.