Until Zonx releases a patch, simply:
Delete eq2ui_mainhud_mail.xml
That will cause the client to pick up the default file, give you the default mail window, and allow you to keep processing your email.
Re-installing Fetish will not fix the problem - SOE slipped in a new eq2ui_mainhud_mail.xml file on 8/16 (or so). They added a new tab, where SPAM email will be sent. And they added a 'version=2.0' field to one of the existing <Page> lines, so the client could detect when you were usiing an old xml file and give you the pop-up.