Apparently, there is a way to archive emails inside nextcloud (https://github.com/nextcloud/mail/issues/22, apparently solved in https://github.com/nextcloud/mail/pull/7355). However, I cannot find any documentation about how this works (is it just about moving emails to an archive folder on the original mailserver, or moving/copying them into a folder on nextcloud?) or can I find an archive button in the nextcloud mail app. Could anyone please point me to where I can find out more? Thank you!
Archiving apears to work on my set up (Nextcloud AIO 14.1.1 beta) with Mail 5.11.5
On my setup, if you hover the cursor over the message in the left hand list of messages until the icon bar appears, and click on the 3 dots, the submenu that appears has the archive option.
From a quick test, this moves the message into the Archive folder, and this is reflected on the mail server.