Use a hidden computed field to monitor the target field for changes. For example, if the field you want to monitor is named "Dept", make a hidden computed field of the same type named "Monitor" with a formula similar to:
@If((Monitor=Dept)|(!@IsDocBeingSaved); @Return(Monitor); @Success); @MailSend(Dept; ""; ""; ""; "Dept changed to yours"; "Doclink:"; [IncludeDoclink]); Dept