Unfortunately, mail forwarders directly cause your email -- and the mail server -- to be blacklisted if the forward is "To:" gmail.com (and Google Apps), HotMail, Yahoo! Mail, and others.
Rather than try to monitor and manage which mail services are problematic, we do not allow forwards.
We check periodically for forwards setup in CPanel and request that you remove them when found. If a forward causes our mail server to be blacklisted it will be removed immediately.
Please note that gmail.com, Google Apps, HotMail and Yahoo! Mail allow you to download email from your local mailbox using POP3 or IMAP. This is not difficult to setup and completely remove the blacklist problem.
With GMail, you can an "Account" to grab your email. (This procedure is similar in Yahoo, HotMail, etc.) Go to the Accounts page in settings and look for Check mail from other accounts (using POP3):. There you can set it up to check your email at regular intervals. You can also send from your GMail account with a "from" address matching your domain. That is setup right above "Check mail ..," in the Send mail as: area.
If you need any assistance, feel free to open a support ticket and we will help in any way we can.