Redirecting email to Gmail, Yahoo mail, Hotmail, etc has been disabled for quite some time now, for us to offer better quality of service on the server email delivery.
All free email service providers control their bandwidth and the number of emails from other email servers around the world.
Setting redirect means there will be A LOT of emails (including spam) being redirected from your account to these free email services, which could cause a temporary traffic block by them.
We hope you understand our intention on such restriction.
Note: If you are using gmail, there is a better way to get the same result by using Google's Check Mail from Other Accounts feature (remote POP3 access) instead. Please refer to this guide for more details.