How should I configure an intranet between Exim4 and ISPConfig 3 mail server?

Hello ISPConfig 3 Experts,
My ISPConfig v3.3 has been running for a few years and it is still running very strong.

Then, these days I have built a new Ubuntu 26.04 VPS for my iot hardware portal. It requires an email service to send alert emails to my future customers.
So I install Exim4 Lite server. It is good enough for local mail. I do not need or want to configure it as a full internet mail server. I plan to use my present ISPConfig 3 email server as the recipient relay host.

Since, these 2 VPS are hosted in the same data centre, they have been assigned extra intranet IP, for example, 10.0.0.x (ispconfig vps) and 10.0.0.y (iot vps).

As in my ISPconfig 3 hosted web sites, I can send out email from within web portal email clients to the outside internet destination in such a way:
user: [email protected]
password: robot_user_password.
smtp host: localhost:25

Then, in my iot VPS web portal, I tested its email client with these smtp setting:
user: [email protected]
password: robot_user_password.
smtp host: 10.0.0.x:25

However, the iot portal web client will report "connection refused."

So, any email server expert can show me some tips to configure my 2 VPS email servers?

 

Easy way is create an e-mail user on ISPConfig system. Then configure that user on the IOT portal as sending e-mail account.
Another way is configure to Exim4 relay host that sends to your ISPConfig box, and allow Postfixt there to receive forwarded e-mail from your IOT box.

 

In situations like this, I like to test the email connectivity at the command line. You can use the 'telnet' command to connect to port 25 on the email server host.
So, on the client system, type:
[ telnet 10.0.0.x 25 ]
If things are working correctly, you will see:
james@bahama:~/bin$ telnet mailserver 25
Trying 52.xx.yy.56...
Connected to mailserver.
Escape character is '^]'.
220 samui.th.net ESMTP Postfix (Debian/GNU)
If this doesn't work, then that will give you a clue as to what the problem is.

 

I have set it up successfully.

1. Add the Exim VPS IP in the Postfix main.cf
   mynetworks = 127.0.0.0/8 [::1]/128 10.0.0.y
2. In the Exim4 VPS, create a Unix user in the Exim mail group. This username is also identical to the username in your target Postfix [email protected]
3. Open both intranet VPS IPs to each other's firewalls. TCP 25 should be used for both side for intranet. TCP 478 does not work.
4. Test and it should work.