If you drop, legitimate senders will never know why your Sales department didn't get backto them on the $1M RFP.
Since Reject happens while the remote MTA is still connected to your box, it needs to deal with the rejection. It doe NOT send back an E-MAIL.
You won't be generating backscatter spam, and you will be letting people who can type know that they messed up.
> connect
220 <your host> ESMTP xxxxx
> hello my.domain
250 Hi there
> mail from:
me@my.domain
250 Mail from accepted
> rcpt to:
not.there@your.host
554 Recipient Address Rejected: <custom message text>
>DATA (to start the message body)
503 5.5.1 DATA without RCPT TO
So the sending system can't send the message body until at least ONE valid recipient is supplied. It's supposed to INTERNALLY generate a bounce back to the envelope sender with the "554 Recipient Address Rejected: <your custom message text>. This happens on the sending MTA's system.
Also, once you enabled Invalid Recipient rejection, you can also tune up Directory Harvest controls.