How to get Thunderbird to archive messages automatically

No matter this is old question, it appears in search results.

And for some time, there are addons for that.

• Awesome Auto Archive – more advanced, with filtering capabilities. Though UI design is rather rubbish, functionality is unmatched. I'm using it and recommend it to others. Despite addons.mozilla.org claims it only works with Thunderbird 24.0 - 32.0, for me it works with Thunderbird 45 and probably will with later ones.

Alternatively,

• AutoArchive – Oldest one, only archives from Inbox, and has separate age settings for starred and labeled mails. Does not work for me on recent Thunderbird versions (addons.mozilla.org says it works with Thunderbird 3.0 - 24.*, and indeed it stopped working for me with ver 38.*).
• AutoArchive Reloaded – modified one, archives from all folders of configured account (not configurable), have additional age setting for unread mails (additional to starred and labeled settings of AutoArchive). Updated to work on later Thunderbird versions.

The filters only run automatically for any new incoming mail.

For those that are already sitting in your inbox, you'll have to manually select the folder and run the filters for them.

Try setting up some kind of hotkey for the action, or test out the Run Filters on Folder add-on.

The "Age in Days" filter isn't used for existing sitting mail, but for when the sender has their email timestamps some time in the past or future. It's handy for when you're away on vacation or similar black holes of time and when you get back to your email, you don't want to bother dealing with anything sent during the time you were incommunicado.

So if you got back after a month and can't be bothered reading anything older than yesterday, this is where that "Age in Days" filter comes in handy as the emails being downloaded are filtered away leaving only yesterday and forward to deal with right now.

Alternatively you could right click on the folder in question, click properties and under retention policy select to delete messages more than 30 days old.