How long will the emails be retained?
Generally, emails of all user accounts are retained as per the default retention policy set by your administrator. In case, custom retention policies are configured, emails that match the criteria are retained for the retention period mentioned in the policy.
Custom retention policies take precedence over the default policy. So, mails are retained by the retention period which is longer. Any mail that is on-hold will be retained even if the email is older than the retention period. Mails on hold are deleted only after the hold is removed.