reject_smtpd.py

from smtpd import SMTPServer
import asyncore

class RejectSMTPServer(SMTPServer):
    def process_message(self, peer, mailfrom, rcpttos, data):
        return '500 Error'

def start_smtpd():
    server = RejectSMTPServer(('127.0.0.1', 1025), None)
    asyncore.loop()

if __name__ == '__main__':
    start_smtpd()

This script implements simple SMTP server which always reject emails.
you can run this script like below.
$ nohup python reject_smtpd.py &