Feature to archive/backup emails from imap server in nextcloud mail system and be able to search them

Is your feature request related to a problem? Please describe.

Many of us are eager to move away from gmail and the likes, but transferring email threads to a new server is difficult and risky, and most alternative imap servers have very limited space. By now even many gmail users are running out of space and need to find a way to move emails out of the gmail server. Nextcloud mail allows us to work with emails from different imap accounts withing nextcloud, but currently there is no possibility to move these emails into nextcloud storage, which is safely backed up.

Describe the solution you'd like

Something similar has been asked many times before (e.g. https://github.com/nextcloud/mail/issues/9551, https://help.nextcloud.com/t/local-imap-mailbox-in-nextcloud/192210, https://help.nextcloud.com/t/nextcloud-mail-downloading-backing-up-emails/88865). The answers have been to enable saving individual mails as .eml files in nextcloud folders, or considering this out of scope. But why is it out of scope? Nextcloud mail is already very powerful for gathering emails from different servers, and especially for searching emails, so it would be really awesome if we could also store those emails on the nextcloud server and make them equally searchable through the mail app. Essentially like the archive function but with the archive being stored on the nextcloud server rather than the respective imap server. Currently existing solutions for backing up or archiving imap emails live outside of nextcloud, so if someone has that need, nextcloud mail becomes useless if it cannot access these emails. If it is possible to put whole threads into an archive folder on an imap server, why not put them in an archive folder in nextcloud?

Describe alternatives you've considered

No response

Additional context

No response

Start by mapping Nextcloud Mail's existing archive and search entry points and how IMAP messages are represented; the issue names no files or tests. Done would require an agreed implementation for storing archived messages in Nextcloud storage while keeping them searchable, with the scope and behavior defined.