If I chose to automatically sync E-mails with Dynamics (e.g. E-Mail from Contacts), will all historic e-mails from my Mailbox be synced with Dynamics, or just new ones?

As the title says, we are thinking about switching to automatic e-mail sync with Dynamics. What we didn't found in the documentation is whether the whole inbox of the user will be synced retrospectively or just new e-mails after changing the sync setting in the personal settings will be synced.