How can a port be filtered from one machine and open from an adjacent machine?

Question

On my home network, I am trying to connect all the devices to the new SMTP (port 25) relay at my new ISP. I can connect to the port from 2 separate machines (laptop via wireless lan, raspberry pi via ethernet), using both sendmail and telnet. The nmap report is as follows:

# nmap rely.wcg.net.au -p 25

Starting Nmap 7.60 ( https://nmap.org ) at 2019-08-19 07:52 AEST
Nmap scan report for rely.wcg.net.au (203.134.11.19)
Host is up (0.021s latency).
rDNS record for 203.134.11.19: 19.11.134.203.sta.m2core.net.au

PORT   STATE SERVICE
25/tcp open  smtp

Nmap done: 1 IP address (1 host up) scanned in 0.10 seconds

From 2 other devices connected to the same network (Asus router running asusWRT and a fileserver running ubuntu), I cannot connect to this port. The failing nmap is as follow:

# nmap rely.wcg.net.au -p 25

Starting Nmap 7.60 ( https://nmap.org ) at 2019-08-19 07:52 AEST
Nmap scan report for rely.wcg.net.au (203.134.11.19)
Host is up (0.66s latency).
rDNS record for 203.134.11.19: 19.11.134.203.sta.m2core.net.au

PORT   STATE    SERVICE
25/tcp filtered smtp

Nmap done: 1 IP address (1 host up) scanned in 11.28 seconds

There is something that I do not understand here. Can you suggest the next debugging steps?

Thanks

Answer 1

I fixed my error. The file server was connecting via a VPN, whereas the functioning machines were connecting directly to the ISP. Traceroute would have informed me of this. The router not connecting was due to DNS issues.