Is there a way to hide the Email button on the mail subtab on a Purchase Order? Tried

jQuery("#newmessage").hide()

both on CS and UE but no success so far ☹️

Try

.css('display', 'none')

@badazzke .hide() 100% works. try to call it with

jQuery( document ).ready( function() {...} )

or

jQuery( window ).on( "load", function() {...})