Simply put, ports are a gateway to connect to an external device for communication. The default port for transmitting email messages is 25. Over a period of time, due to some security reasons, two more ports, i.e., 587 and 465 were used to transmit email messages. Recently, most of the Internet Service Providers (ISP) are blocking port 25 to control spamming. For business purposes, they configure their firewalls to allow only port 25 from the designated mail server. port 25 is not a secure option for transmitting email messages. And, the same is used by the spammers too so, you have to fix the SMTP port 25 security risk.
When your computer is infected with a virus, it is very easy for the hijackers to send n number of spam emails. The only solution for this is that ISP has to block the port. If you want to check whether port 25 is blocked by your ISP, enter the ‘telnet example.com 25’ command in the Terminal window.
If you see the 220 response, it indicates that port 25 is not blocked.
If port 25 is blocked by your ISP, you will see the ‘Unable to connect to remote host’ message in the Terminal window.
It is better to use the port 465 or 587 instead of 25. Both the 587 and 465 ports are secure for email transmission.