Ever had an e-mail address that keeps popping up in your autocomplete field in Thunderbird? If you've used other e-mail clients in the past, usually you can select the entry using the keyboard and hit either [DELETE] ( or [SHIFT]+[DELETE] like you can in Firefox.)
Apparently Thunderbird compiles the autocomplete list not from a cache, but from e-mail addresses in your Address Book or by guessing for users in your domain. So why is that rogue invalid entry in there you ask? Well, perhaps it was added to your Address Book automatically under the "Collected Addresses" section. That's what happened to me.
All of the sudden a name started appearing in the drop down list that was invalid. It was causing me problems because it was always popping up as the first choice when trying to e-mail my boss. So, if you're seeing some invalid e-mail addresses showing up in your autocomplete list in Thunderbird, do the following:
Now when you go to type in an e-mail address, the invalid entries should no longer appear!