Add filtermail

filtermail/filtermail.service

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+[Unit]
+Description=Email filter for chatmail servers
+
+[Service]
+ExecStart=/usr/local/bin/filtermail
+Restart=always
+RestartSec=30
+
+[Install]
+WantedBy=multi-user.target

filtermail/pyproject.toml

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+[build-system]
+requires = ["setuptools>=45"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "filtermail"
+version = "0.1"
+dependencies = [
+  "aiosmtpd"
+]
+
+[project.scripts]
+filtermail = "filtermail.filtermail:main"
+
+[tool.pytest.ini_options]
+addopts = "-v -ra --strict-markers"
+
+[tool.tox]
+legacy_tox_ini = """
+[tox]
+isolated_build = true
+envlist = lint
+
+[testenv:lint]
+skipdist = True
+skip_install = True
+deps =
+  ruff
+  black
+commands =
+  black --quiet --check --diff src/
+  ruff src/
+"""

filtermail/src/filtermail/filtermail.py

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+#!/usr/bin/env python3
+import asyncio
+import logging
+
+from aiosmtpd.lmtp import LMTP
+from aiosmtpd.controller import UnixSocketController
+from smtplib import SMTP as SMTPClient
+
+
+def check_encrypted(envelope):
+    """https://xkcd.com/1181/"""
+    return "-----BEGIN PGP MESSAGE-----" in envelope.content.decode(
+        "utf8", errors="replace"
+    )
+
+
+class ExampleController(UnixSocketController):
+    def factory(self):
+        return LMTP(self.handler, **self.SMTP_kwargs)
+
+
+class ExampleHandler:
+    async def handle_RCPT(self, server, session, envelope, address, rcpt_options):
+        envelope.rcpt_tos.append(address)
+        return "250 OK"
+
+    async def handle_DATA(self, server, session, envelope):
+        logging.info("Processing DATA message from %s", envelope.mail_from)
+
+        valid_recipients = []
+
+        mail_encrypted = check_encrypted(envelope)
+
+        res = []
+        for recipient in envelope.rcpt_tos:
+            my_local_domain = envelope.mail_from.split("@")
+            if len(my_local_domain) != 2:
+                res += [f"500 Invalid from address <{envelope.mail_from}>"]
+                continue
+
+            if envelope.mail_from == recipient:
+                # Always allow sending emails to self.
+                valid_recipients += [recipient]
+                res += ["250 OK"]
+                continue
+
+            recipient_local_domain = recipient.split("@")
+            if len(recipient_local_domain) != 2:
+                res += [f"500 Invalid address <{recipient}>"]
+                continue
+
+            is_outgoing = recipient_local_domain[1] != my_local_domain[1]
+            if is_outgoing and not mail_encrypted:
+                res += ["500 Outgoing mail must be encrypted"]
+                continue
+
+            valid_recipients += [recipient]
+            res += ["250 OK"]
+
+        # Reinject the mail back into Postfix.
+        if valid_recipients:
+            logging.info("Reinjecting the mail")
+            client = SMTPClient("localhost", "10026")
+            client.sendmail(envelope.mail_from, valid_recipients, envelope.content)
+
+        return "\r\n".join(res)
+
+
+async def asyncmain(loop):
+    controller = ExampleController(
+        ExampleHandler(), unix_socket="/var/spool/postfix/private/filtermail"
+    )
+    controller.start()
+
+
+def main():
+    logging.basicConfig(level=logging.INFO)
+    loop = asyncio.new_event_loop()
+    asyncio.set_event_loop(loop)
+    loop.create_task(asyncmain(loop=loop))
+    loop.run_forever()
+
+
+if __name__ == "__main__":
+    main()