While not recommended, it is possible to migrate emails from another server (gmail).

After settings up the mailbox by an admin, it is possible fetch on a per-mailbox basis via:

sudo freescout-manage freescout:fetch-emails --days=$days --unseen={0,1} -mailboxes=$mailbox_id

If unseen=0, it will fetch all emails. If unseen=1, it will fetch only unread emails.

After migration, the emails will swarm as if they came in today, to import according to dates that were actually received, you can set, in FreeScout's config file, the following setting:

APP_USE_MAIL_DATE_ON_FETCHING=true 

It is important to keep this set to false by default.

To clear-out a mailbox, you can use the following commands:

sudo freescout-manage cleanup:conversations --mailbox-id=[MAILBOX_ID] --older-than-days=0 --y