SMTP Debugging/QA/Testing Server, Simulate easily SMTP errors

async-smtp-test-server.py

from aiosmtpd.controller import Controller
import asyncio


class ExampleHandler:
    async def handle_RCPT(self, server, session, envelope, address, rcpt_options):
        print(f"handle_RCPT: {address}")
        if address.endswith('@error.com'):
            user_part = address.split("@")[0]
            try:
                # Send recipient like this: 550-the-email-account-that-you-tried-to-reach-is-disabled.@error.com and get error: 
                # SMTPRecipientsRefused 
                # {'550-the-email-account-that-you-tried-to-reach-is-disabled.@error.com': (550, b'The Email Account That You Tried To Reach Is Disabled.')}
                msg = " ".join(user_part.split('-')).title()
                print(msg)
                return msg
            except:
                print("Failed to parse error email")
                msg = '550 not relaying to that domain'
                print(msg)
                return msg
        envelope.rcpt_tos.append(address)
        msg = '250 OK'
        print(msg)
        return msg

    async def handle_DATA(self, server, session, envelope):
        print('Message from %s' % envelope.mail_from)
        print('Message for %s' % envelope.rcpt_tos)
        print('Message data:\n')
        for ln in envelope.content.decode('utf8', errors='replace').splitlines():
            print(f'> {ln}'.strip())
        print()
        print('End of message')
        return '250 Message accepted for delivery'


controller = Controller(ExampleHandler(), port=1025)
controller.start()
input("Server started. Press Return to quit.\n")
controller.stop()

# Output:
# Server started. Press Return to quit.
# handle_RCPT: 550-the-email-account-that-you-tried-to-reach-is-disabled.@error.com
# 550 The Email Account That You Tried To Reach Is Disabled.
# handle_RCPT: 550-temporary-server-error@error.com
# 550 Temporary Server Error
# handle_RCPT: normal@example.com
# 250 OK
# Message from a@a.com
# Message for ['normal@example.com']
# Message data:

# > From: A <a@a.com>
# > To: B <b@b.com>
# > Subject: SMTP e-mail test
# >
# > Hello World!

# End of message

simple-smtp-test-client.py

import smtplib
import time

sender = 'a@a.com'
recipients = [
    "550-the-email-account-that-you-tried-to-reach-is-disabled.@error.com",
    "550-temporary-server-error@error.com",
    "normal@example.com",
]

message = """From: A <a@a.com>
To: B <b@b.com>
Subject: SMTP e-mail test

Hello World!
"""

for r in recipients:
    try:
        smtp = smtplib.SMTP('localhost', 1025)
        smtp.sendmail(sender, [r], message)         
        print("Successfully sent email")
    except smtplib.SMTPException as ex:
        print(ex.__class__.__name__, str(ex))
        time.sleep(0.1)

# Output: 
# SMTPRecipientsRefused {'550-the-email-account-that-you-tried-to-reach-is-disabled.@error.com': (550, b'The Email Account That You Tried To Reach Is Disabled.')}
# SMTPRecipientsRefused {'550-temporary-server-error@error.com': (550, b'Temporary Server Error')}
# Successfully sent email


def text_to_username(text):
    return f'550-{"-".join(text.lower().split())}@error.com'


text_to_username("Temporary server error")

# Output:
# '550-temporary-server-error@error.com'