Wolkom by Thunderbird Add-ons.
Foegje ekstra funksjes en stilen ta om Thunderbird hielendal fan jo te meitsjen.
Slute
Single Domain 4.3
troch Graeme
Warns if you send mail from or to a specified domain. If there are multiple accounts in your profile, you can isolate one so that you can't send mail to that domain from outside or vis versa without warning.
Oer dizze add-on
As of TB 91 there is a bug that blocks sending if there is no dictionary added. Add a dictionary and everything should unlock.