Forwarding emails to an external provider can lead to account suspension if you receive a lot of spam to your email address. All spam that arrives at your mail address will be forwarded including spam and can get the server blacklisted.
Also forwarding email in this fashion is frowned upon by the big email providers and blocked, so genuine emails will be block indiscriminately with no reliability of delivery.
Forwarding to internal email boxes is what this was designed to do and is 100% reliable.
Any mailbox abusing these rules could be throttled at account level, suspended and in all cases the forwarder will be removed with no notice as its automated.
Birch Hosting Ltd