OK, so I have a central GMail account, and I have several e-mail addresses forwarding to this account. I also use this account to send e-mails from these other accounts. However, I get an error e-mail saying that my central gmail account isn't registered, so I can't send the message. But I have set my e-mail address to the accepted address!

If this can't be changed, you'd better warn people about it. You can't use gmail to send e-mails from other e-mails that you own.