To Display Custom Headers in Opera 12 Mail
Open an email
Right-click one of the currently displayed header values and goto
Click add and Enter the custom header (like X-OpenGroupware-Regarding)
(From the Opera
Previous Opera versions you would right-click in the email header box to
get a dialog box to choose which items would be displayed in the header.
In Opera 12, you right-click on a header (like the From address) in the
header bar above the message.. You can't just right-click anywhere on the
header bar to get it.
Using Opera's revolutionary email client: http://www.opera.com/mail/