I purchased a new router (TP-Link ER6020), turned it on and connected my computer to it. The router is running in NAT mode, I did not change a setting (despite providing dsl authentication details). Then I ran a port scan using an online port scanner. It found one port (port 25 - SMTP), that was open (from WAN side).
I thought with NAT by default no port should be available from the WAN side (port 80 for config only from LAN). Am I wrong there? Any idea how I can and whether I need to fix this (I found no corresponding setting in the router)?
Edit: Using telnet I can connect to port 25 (from LAN!). But the router is not responding to the HELO command. And after a few seconds I get:
network error: software caused connection abort
and then
421 Cannot connect to SMTP server <ip-address> (<ip-address>:25), NB connect error 1460