aiosmtpd is an excellent tool for writing custom routing and header rewriting rules for email. However, aiosmtpd is not an MTA, since it does not do message queuing or DSN generation. One popular choice of MTA is postfix, and since postfix can be configured to relay all emails for a domain to another local SMTP server (such as aiosmtpd), a natural choice is to use postfix as the internet-facing frontend and aiosmtpd as the business-logic backend.
Advantages of using postfix as the middle-man instead of letting aiosmtpd face the public internet:
- No need to handle DNS MX lookups in aiosmtpd -- just relay through postfix (localhost:25)
- No worry about non-compliant SMTP clients in aiosmtpd
- No worry about STARTTLS in aiosmtpd -- configure this in postfix instead (simpler and more battle-hardened)
- No worry about retrying failed email deliveries and sending delivery status notifications
- aiosmtpd can be configured to respond with "transient failure" (SMTP 4xx code) upon programming errors, so no email is lost as long as the programming error is fixed within 4 days
Here's how you might configure postfix to work with a local SMTP server powered by e.g. aiosmtpd.
We're going to run postfix on port 25 and aiosmtpd on port 20381.
To specify that postfix should relay emails for example.com to an SMTP server running on port 20381, add the following to /etc/postfix/main.cf:
transport_maps = hash:/etc/postfix/smtp_transport
relay_domains = example.com
And create /etc/postfix/smtp_transport with the contents:
# Table of special transport method for domains in
# virtual_mailbox_domains. See postmap(5), virtual(5) and
# transport(5).
#
# Remember to run
# postmap /etc/postfix/smtp_transport
# and update relay_domains in main.cf after changing this file!
example.com smtp:127.0.0.1:20381
Run postmap /etc/postfix/smtp_transport after creating that file (and every time you modify it).
On the aiosmtpd side, there are a few things to consider.
The most important is how you handle bounce emails. The short story is that you should set the envelope sender to an email address you control that is dedicated to receiving bounces, e.g. bounce@example.com. When email arrives at this address, it should be stored somewhere so you can process bounces, e.g. by removing member email addresses from your database.
Another important thing to consider is how you tell your members' email providers that you are doing mailing list forwarding. You might want to add the following headers when forwarding emails to GROUP@example.com:
Sender: bounce@example.com
List-Name: GROUP
List-Id: GROUP.example.com
List-Unsubscribe: <mailto:postmaster@example.com?subject=unsubscribe%20GROUP>
List-Help: <mailto:postmaster@example.com?subject=list-help>
List-Subscribe: <mailto:postmaster@example.com?subject=subscribe%20GROUP>
Precedence: bulk
X-Auto-Response-Suppress: OOF
Here, I used postmaster@example.com as the recipient for list unsubscribe requests. This should be an address that forwards to the email administrator (that is, you).
Below is a skeleton (untested) that does the above. It stores bounce emails in a directory named bounces and forwards emails with a valid From:-header (one that appears in MEMBERS) according to the list of groups (in GROUPS).
import os
import email
import email.utils
import mailbox
import smtplib
import aiosmtpd.controller
LISTEN_HOST = '127.0.0.1'
LISTEN_PORT = 20381
DOMAIN = 'example.com'
BOUNCE_ADDRESS = 'bounce'
POSTMASTER = 'postmaster'
BOUNCE_DIRECTORY = os.path.join(
os.path.dirname(__file__), 'bounces')
def get_extra_headers(list_name, is_group=True, skip=()):
list_id = '%s.%s' % (list_name, DOMAIN)
bounce = '%s@%s' % (BOUNCE_ADDRESS, DOMAIN)
postmaster = '%s@%s' % (POSTMASTER, DOMAIN)
unsub = '<mailto:%s?subject=unsubscribe%%20%s>' % (postmaster, list_name)
help = '<mailto:%s?subject=list-help>' % (postmaster,)
sub = '<mailto:%s?subject=subscribe%%20%s>' % (postmaster, list_name)
headers = [
('Sender', bounce),
('List-Name', list_name),
('List-Id', list_id),
('List-Unsubscribe', unsub),
('List-Help', help),
('List-Subscribe', sub),
]
if is_group:
headers.extend([
('Precedence', 'bulk'),
('X-Auto-Response-Suppress', 'OOF'),
])
headers = [(k, v) for k, v in headers if k.lower() not in skip]
return headers
def store_bounce_message(message):
mbox = mailbox.Maildir(BOUNCE_DIRECTORY)
mbox.add(message)
MEMBERS = ['foo@example.net', 'bar@example.org',
'clubadmin@example.org']
GROUPS = {
'group1': ['foo@example.net', 'bar@example.org'],
POSTMASTER: ['clubadmin@example.org'],
}
class ClubHandler:
def validate_sender(self, message):
from_ = message.get('From')
if not from_:
return False
realname, address = email.utils.parseaddr(from_)
if address not in MEMBERS:
return False
return True
def translate_recipient(self, local_part):
try:
return GROUPS[local_part]
except KeyError:
return None
async def handle_RCPT(self, server, session, envelope, address, rcpt_options):
local, domain = address.split('@')
if domain.lower() != DOMAIN:
return '550 wrong domain'
if local.lower() == BOUNCE:
envelope.is_bounce = True
return '250 OK'
translated = self.translate_recipient(local.lower())
if translated is None:
return '550 no such user'
envelope.rcpt_tos.extend(translated)
return '250 OK'
async def handle_DATA(self, server, session, envelope):
if getattr(envelope, 'is_bounce', False):
if len(envelope.rcpt_tos) > 0:
return '500 Cannot send bounce message to multiple recipients'
store_bounce_message(envelope.original_content)
return '250 OK'
message = email.message_from_bytes(envelope.original_content)
if not self.validate_sender(message):
return '500 I do not know you'
for header_key, header_value in get_extra_headers('club'):
message[header_key] = header_value
bounce = '%s@%s' % (BOUNCE_ADDRESS, DOMAIN)
with smtplib.SMTP('localhost', 25) as smtp:
smtp.sendmail(bounce, envelope.rcpt_tos, message.as_bytes())
return '250 OK'
if __name__ == '__main__':
controller = aiosmtpd.controller.Controller(ClubHandler, hostname=LISTEN_HOST, port=LISTEN_PORT)
controller.start()
print("Controller started")
try:
while True:
input()
except (EOFError, KeyboardInterrupt):
controller.stop()
