mailserver not allowed to use port 25

[sir-andreas]

When I start the server as docker I get the following error in the mailserver.log:

PermissionError: [Errno 13] error while attempting to bind on address ('0.0.0.0', 25): permission denied

Port 25 is free on the server and can be used by other docker containers without issues.

[boraxhacker]

I got the same error (below). Looking at rootfs/start.sh and noticing nginx as a user which probably can't use lower ports, I set environment variable, MAILPORT, to a high port, 25025 and mapped outside port 25 to inside 25025.

su - nginx -s /bin/ash -c 'cd /var/www/opentrashmail/python;python3 -u mailserver3.py

cat mailserver.log
2025-06-01 02:59:19,692 - main - INFO - [i] Discard unknown domains: False
2025-06-01 02:59:19,692 - main - INFO - [i] Max size of attachments: 0
2025-06-01 02:59:19,692 - main - INFO - [i] Listening for domains: ['localhost']
Traceback (most recent call last):
File "/var/www/opentrashmail/python/mailserver3.py", line 304, in
asyncio.run(run(port))
File "/usr/lib/python3.11/asyncio/runners.py", line 190, in run
return runner.run(main)
^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/asyncio/runners.py", line 118, in run
return self._loop.run_until_complete(task)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
return future.result()
^^^^^^^^^^^^^^^
File "/var/www/opentrashmail/python/mailserver3.py", line 240, in run
controller_plaintext.start()
File "/usr/lib/python3.11/site-packages/aiosmtpd/controller.py", line 274, in start
raise self._thread_exception
File "/usr/lib/python3.11/site-packages/aiosmtpd/controller.py", line 235, in _run
self.server = self.loop.run_until_complete(self.server_coro)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/asyncio/base_events.py", line 654, in run_until_complete
return future.result()
^^^^^^^^^^^^^^^
File "/usr/lib/python3.11/asyncio/base_events.py", line 1536, in create_server
raise OSError(err.errno, msg) from None
PermissionError: [Errno 13] error while attempting to bind on address ('0.0.0.0', 25): permission denied