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Tested only twice but seems to work pretty well under TB v.115.8. There is a loader upon editing subject, probably related to saving email in temporary 'EES-temp' local folder before renaming it. It last 2 seconds max for me on an old computer. Still surprising to have this loader indicating some slow process, even for 2 seconds, but all in all, up to now, this seems to work well and as expected.

However, as someone below noted, it seems to work only in the inbox, not in other folders. In other folders it appears to yied an error. However when refreshing the given folder, subject editing actually worked.

I'm also just surprised this is a standard Thunderbird feature (or a default pre-installed add-one) and that an add-on would be required to be installed for this.

I am trying to resolve the long-wait time. The technical background here is that the entire message is modified, and not just an entry in a Thunderbird DB. If you have multiple email clients (on your phone, tablet and maybe a second Thunderbird somewhere else), they all get the updated subject. That however means I have to wait for your IMAP server to ack the change, and that is what takes so long. Renaming a message in a local folder should be instant.

Some email servers (for example google), ignore such update requests and TB never gets the ack from the server. I am trying to fix that, but have not found a handle yet.

As for "why is this not a default feature". The idea of add-ons is that users can enhance Thunderbird with features they need. Who decides what is a basic function and should be part of core? That is a tough one. We have 10M+ users and everybody has different needs. If we add all the add-ons which someone considers as "basic", we would have no more add-ons and TB would be a over-complex product with a crowded UI (we already have that issue now).