Sobre esta extensão
https://hogiblog.blogspot.com/2020/09/mailextensions-priority-switcher.html
Change priority of the received messages from the toolbar button and the context menus.
Notice that this extension changes priority in the *msf files. So, if you reconstruct the files, all changes will be gone.
* From 0.4.10, you can commit the change of priority to X-Priority. This rewrites original message so the change will not be gone if you commit. Moreover, you can share the changes on all environments if you use IMAP.