I have migrated my mailcow instance to a new server and have use the corresponding helper scripts to backup and restore mails and configs etc.
The admin interface shows for account all old mails (They indeed seem to there, at least I check the dovecot directories in /var/vmail/. But I don’t use SoGo to check there as well).
However, when I fetch then using thunderbird, no old mails are shown.
If I send a test mail to the server, the new mails show ups. Meaning, new mails are shown but all old mails are not.