I'm tired of mass forwarded messages and want to delete any future email that meets a criteria such as having more than 10 recipients in the TO: or CC: headers.
How do I set up such a filter in in Thunderbird?
You should be able to create a rule/filter in Thunderbird, as far as I know...I don't have it myself, but here is a tutorial on:
If you can't delete mails based on a rule/filter, I suggest just filtering out all messages with 10+ contacts and putting them in a separate folder, and then just mass-delete everything in the folder from time to time...