Over time Mozilla’s Thunderbird may happen to require some maintenance in order to continue running smoothly. Symptoms could be that either you cannot find mails that are supposed to be there, mails are doubling without any reason, or deleted mails show up again in its original folder.
Note that Thunderbird must be closed (and really not running) for the next steps to be successful. You then need to navigate to your profile folder and execute the given commands at the root of this directory.
As more and more people want to encrypt their personal communication to preserve and to protect their privacy, this post should give a quick introduction and some useful hints regarding email encryption based on OpenPGP.
First of all: It may initially look like being quite complicated. The available guide will require some thorough reading. But in the end it is not that complex and absolutely worth the effort. As soon as one is familiar with the basic concepts and techniques it will be nothing more than locking/unlocking your front door. And probably you did not yet remove your door lock for reasons of simplicity?