If your domain is example.com and you want to forward your email from email@example.com to firstname.lastname@example.org or any other domain that is not hosted by us, this is external forwarding. This can be very dangerous because spam can be forwarded. This gives our mail server a bad reputation because the receiving mail server sees us as the source instead of the original source.
There is a safe and better way! Most large mail providers have an option to check an external mail account through POP3 or IMAP. You use the same settings as if you were setting up the account in Outlook/Thunderbird or any other mail app. Here are links to some tutorials for popular mail providers:
If this is not an option for you, please contact us and we can help you to find the right solution.
Any other type of forwarding falls under internal forwarding and you can view the documentation here: https://documentation.cpanel.net/display/ALD/Forwarders