"Please, if I send you an email labeled as urgent, I expect you to treat it urgently and respond/reply (to it) as soon as you could." Instead of "could", you could also use "can" or "as soon as possible" or "as soon as you're able to". I think "could" sounds a bit awkward. "You may stop working on whatever you're working on and reply to/respond to/do it right away. Then you can resume your previous assignment after replying/responding (to it)." You could make them two separate sentences or join them "...right away, and then..." "If you could rephrase it to sound more professional, I'll be indebted to you." Are you a boss, employer, manager, or in some other position of authority?

"Please, if I send you an email labeled as urgent, I expect you to treat it as such and submit it as soon as you can. You must stop working on whatever you're working on and do it right away then you can resume your previous assignment after submitting it." So harsh haha