This is a common problem with users whose e-mail address has changed. When an e-mail address has changed, even slightly, the list server will no longer recognize you even if your mail is being forwarded.
This happens when your e-mail address has changed so the daemon no longer recognizes you as you. You must be subscribed to the list to post, and if your current e-mail address is not registered with the daemon it won't approve it. Just because the mail is getting to you doesn't mean the list knows who you are.
This is the reason we encourage users to update their mailing list subscriptions when their addresses change rather than relying on mail forwarding for the mail to get there. That can create all sorts of problems in the future, long after you've forgotten which address you subscribed to a given list with.
To gain the ability to post, you will have to unsubcribe the old address, or ask the admins to unsubscribe it for you, and then resubscribe to the mailing list with your new e-mail address.
See the FAQ item on My e-mail address has changed for how to fix this problem.