I need help

1. Unraid 7.2.4 host

2. Ubuntu VM bridged on br0

3. Windows host on same LAN can connect to Gmail MX on TCP/25

4. Unraid host and Ubuntu VM cannot connect to any remote SMTP server on TCP/25

5. Port 587 works

6. DNS, routing, iptables, and router configuration appear normal

7. Looking for anyone who's seen outbound TCP/25 blocked only from the Unraid host

I am not sure this will answer your question BUT google security issue on opening port 25 and read the AI summary.

Apparently, the current recommendation is to use port 25 only as a last resort. see below:

I also checked to see what port that I had my Unraid e-mail Notifications for. It was using port 465. (This setting is the default for the Yahoo mail server.)

This is a bit late but:

Port 25/TCP is only used for incoming mail as standard (Simple Mail Transfer Protocol (SMTP),used for email routing between mail servers)

Port 465/TCP preferred port for Message Submission over TLS protocol.

Port 587/TCP Email Message Submission STARTTLS (No longer preferred; use port 465)

I get most of my port related info from here (https://en.wikipedia.org/wiki/List_of_TCP_and_UDP_port_numbers)

just a note; never use TCP/UDP for all incoming ports for security reasons, try finding the proper protocol and assign it.