Does rejecting spam cause additional SMTP traffic?
It does, but not directly for you. The only difference between accepting and rejecting a mail is your side’s reply to the DATA command: In the accept case it is “250 message queued for delivery” (or something similar), in the reject case it is “550 [your rejection message]”. The mail delivery failure notice is not created by your mail server, but by the sending mail server. The mail delivery failure notice typically does not reach spammers at all, as a spammer won’t give his real address, but it can be useful to inform legitimate users that their message was tagged as spam and might not be read.
