If you want to keep them, why do you delete them in the first place? As per your screenshot, you're asked to let Thunderbird compacting folders. Compacting is necessary maintenance to keep you installation of Thunderbird going, see https://thunderbirdtweaks.blogspot.com/2011/07/compacting-what-is-it-and-why-must-i-do.html and http://kb.mozillazine.org/Compacting_folders This is not about emptying the Trash, which is an entirely different thing. Having said that, with large folders there's an increasing risk that mail files get corrupted. Therefore you should create a full backup of your Thunderbird profile. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data#w_backing-up-a-profile Note, since you didn't let Thunderbird compact folders in a while, the backup will be 2.5 GB larger than it needs to be. Also, your screenshot indicates you're running a daily version of Thunderbird, which is an alpha version and literally updates daily. It is solely intended for testing purposes, and should not be used on production systems. therefore, why would you want to run the daily version, and not the release version? The release version currently is 68.6.0.

I will do all this tomorrow including updating Thunderbird to the release version. I didn't realise that the version I installed was testing version. Many thanks for your help. Mike-on-the-Costa