Postfix SMTP on Remote Server Responds 554 Relay Access Denied for rcpt (recipient). Works okay on 'localhost' smtp server

Question

I'm trying to send e-mail through a remote vm running postfix as an smtp server.

However, I'm receiving a 554 - Relay access denied error when sending the rcpt cmd.

(554, b'5.7.1 <xxxxx@xyz.org>: Relay access denied')

I understand that it is denying the e-mail address I'm attempting to set as a recipient, but I don't understand why.

The socket connection itself establishes fine, and I'm able to send a mail cmd prior to attempting rcpt.

Environment Details:

Host Machine:

Windows 7 machine / Language: Python 3.

Guest VM:

Docker container (Ubuntu) VM running postfix, ssh -- with dhcp-assigned ip address 10.35.50.166.

This is the python code running:

(on host machine):

import smtplib
server = smtplib.SMTP(host='10.35.50.166', port=8025)

server.mail('xxxxx@xyz.org')
(250, b'2.1.0 Ok')

server.rcpt('xxxxx@xyz.org')
(554, b'5.7.1 <xxxxx@xyz.org>: Relay access denied')

Running from the guest machine, the error does not occur:

import smtplib
server = smtplib.SMTP(host='localhost', port=8025)

server.mail('xxxxx@xyz.org')
(250, b'2.1.0 Ok')

server.rcpt('xxxxx@xyz.org')
(250, b'2.1.5 Ok')

Answer 1

Postfix' default configuration on Ubuntu allows relay access only on the local interface (i.e. localhost). When connecting on other interfaces (VM network), it requires SMTP authentication.

So you could either add authentication to your SMTP call or add your host ip to the allowed relay networks.

To achive the later, find the line in your /etc/postfix/main.cf file that reads

mynetworks = 127.0.0.0/8

and change it to

127.0.0.0/8,10.0.0.0/8

Then reload postfix with

sudo postfix reload

Edit: Alternatively, you could set the mynetworks_style setting to

mynetworks_style = subnet

See BASIC CONFIGURATION README