If your account connects using POP, then deleting server copies when you download messages is the default behaviour, unless you specifically change it by setting the checkbox "Leave a copy on server".This can be found under Account Settings.

If your account uses IMAP then it will by its nature leave copies on the server. If you want to clear the server, move the messages you want to keep to the Local Folders account in Thunderbird. Doing this will erase the server copies. For messages you don't want to keep, just delete them in Thunderbird. IMAP synchronization will delete the server copies too.

Comment importer une clé publique OpenPGP dans Thunderbird ?
https://kb.mailfence.com/fr/kb/importer-cle-publique-openpgp-dans-thunderbird/

How to Encrypt Emails with OpenPGP in Thunderbird
https://www.linuxbabe.com/security/encrypt-emails-gpg-thunderbird

See the revision history at the end of this document. Thunderbird is a very popular, free, open source, multi-platform, extensible email client with included calendar functionality and encryption.

L'extension "Send Later" pour Thunderbird permet de planifier l'envoi d'emails à un moment précis ou récurrent, offrant des options comme "15 minutes plus tard". Elle utilise Sugar.js pour le parsing de dates et est soutenue par Extended Thunder Inc., une organisation à but non lucratif.

If you use multiple email accounts in Thunderbird, you probably want to set up separate email signatures. Here's how to do so with a popular add-on.

Configuration de Thunderbird - Zaclys