It kind of bugs me when I see people write “please reply by return” in e-mails. Do people actually think about whether what they write makes sense? Or do they just write things which they think are official sounding?
I suspect many use this phrase because they may have seen similar language on business letters of old. What they may not realise is that in the past, when companies still depended on either snail mail or fax for written communication, it makes sense to state “please return by post” to indicate that snail mail is required/ preferred over fax.
When it comes to e-mail, however, a reply is what it is. What else is there to “return”? If you specifically wanted someone to give you an answer over e-mail (versus a phone call or fax or something), then saying “please let me know your answer via e-mail” or “please reply via e-mail” would make a whole lot more sense!