How do I fetch emails created before the accounts was connected in V3, historical sync?

Unlike API V2 we do not sync objects as such you will need to make the API calls and paginate through the cursors of the /v3/grants/{grant_id}/messages endpoint.

You can use this method to fetch all the emails or for just a specific time period as per V2.

Please note that this can hit rate rate-limiting on the provider side, as such space our your API calls.