Opera

It is not possible to completely switch off the header when viewing an e-mail, but you can remove all fields that you don't need. If you switch off all fields, the header will only be visible as a thin ridge between the e-mail view and the mailbox view.

To choose which header fields you want to see, right-click in the header area outside of the individual headers, and choose Customize. In the dialog box "Customize Mail Headers", uncheck the fields you want to remove, and click OK. You can always go back to the dialog box and check the field that you want.