Staff forwarding business mail to a personal Gmail account is common and usually a bad idea.
WHY PEOPLE DO IT
Convenience. One inbox on their phone rather than two.
WHY IT IS A PROBLEM
Business correspondence, including customer personal data, leaves your control and sits in an account you cannot access, audit or recover. When the person leaves, the mail goes with them. The forwarding often breaks authentication, so forwarded mail lands in spam and the person assumes the sender never wrote. If their personal account is compromised, your business correspondence is exposed. It may conflict with your obligations over customer data.
WHAT TO DO INSTEAD
Set the business mailbox up properly on their phone using IMAP. It takes five minutes and solves the underlying convenience problem.
POLICY
State plainly that business mail is not to be auto-forwarded to personal accounts, and say why. Most people forward out of convenience rather than defiance, and stop when shown the alternative.
CHECKING
cPanel > Forwarders lists every forwarder on the account. Review it occasionally.
