[enhancement]: re-filter all existing mails by sieve scripts

[liubog2008]

Which feature or improvement would you like to request?

I hope to re-filter all existing mails by sieve scripts. Now mails can only be filtered at delivery time.

Is your feature request related to a problem?

I use isync to fetch mails from remote and sync to my local stalwart mail server. However, sieve script cannot worked for this case now.

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