I have not seen this expression myself - if you wanted someone to reply to your email and felt that you needed to say so in the email, you would write "please reply" or " I would appreciate a reply to this email". I would guess that the person who used this expression may not be familiar with English and is trying to say "reply to" but using the wrong word. Perhaps others on Italki have seen this expression but I haven't and would not recommend you use it.

Returning an email is different from replying to or answering an email. 'Return' means 'send back'. If you receive a letter that's not for you, you send it back marked 'Return to sender'. If you buy goods that aren't suitable, you return them to the supplier. If you write a reply to an email and then send it, this is not 'returning' it. It's answering or replying. The only time you would 'return' an email, as far as I can see, is if it is unsolicited (spam or similar) and you simply click 'return'. Here's some advice on why you shouldn't do this: http://www.outlook-tips.net/beginner-user/return-email-to-sender/