Port 25 is the standard port which mail servers listen on. Many ISP's block inbound and outbound port 25 to stop spammers from abusing their networks. This makes running a mail server on one of these ISP's networks nearly impossible.

Inbound vs Outbound ports? - LinuxQuestions.org Feb 10, 2008 [SOLVED] "opening ports" vs port forwarding? - Firewalls Jul 20, 2016 Sonicwall Port Forwarding and LAN WAN Rules Basics

Single Port Forwarding will only allow you to specify one port for forwarding. If you want to forward multiple ports, you can perform Port Range Forwarding instead. Click here for instructions. NOTE: If you want to set up Single Port Forwarding through your Linksys server account, click here to learn how. If you are replacing an old router with

Linksys Official Support - Setting up Single Port Single Port Forwarding will only allow you to specify one port for forwarding. If you want to forward multiple ports, you can perform Port Range Forwarding instead. Click here for instructions. NOTE: If you want to set up Single Port Forwarding through your Linksys server account, click here to learn how. If you are replacing an old router with

[SOLVED] "opening ports" vs port forwarding? - Firewalls

Consumers - TG862G/NA: Port Triggering Setup