I don't know that you can tag an outgoing mail and have that ''automatically'' persist in a reply. The more usual way of doing this is by message 'threads': https://support.mozilla.org/en-US/kb/configuration-options-viewing-messages When you enable this, an algorithm in Thunderbird tries to group messages together when they are part of an ongoing conversation. I don't know how it does it, and in my experience it's not 100% reliable, but try it and see. You can always switch back to another display. There's also an add-on called 'Thunderbird Conversations' which might do what you want. Of course, if all the mails relating to a specific project quote a project reference (phrase or number), then you could simply search for that and put all those emails in a folder as a saved search: https://support.mozilla.org/en-US/kb/using-saved-searches So there are several possibilities ... try them out and see if any work for you.