Sorry, but there is no story here, really. In fact the exchange was kind of a lame attack on Garrett's part.
Major gave no specifics of the content of the emails and without more specifics, there is no way Gibbs could answer his question, even if he was an email expert. Those emails could have come from anywhere and still have had whitehouse.gov as the sender. SMTP servers do not care what is placed in the senders "from" address. You can put anything there and the server will send the email. Garrett should have done more homework before attacking on this issue.