On the new iPhone 3g, how do I create new mailboxes in which to save emails I get.  Also, how do I save attachments? I received a word doc attachment but there is no way I can save it?

I just got my iphone and set up my email account with Earthlink, but since it is through Roadrunner, I cannot reply to any email.  The Apple Genious told me to set up an Gmail account and have my email forwarded.  I did this, but the iphone will not recognize my gmail address as authentic and keeps telling me to enter a valid Gmail account.  any ideas on how to fix this problem?