Explaining SMTP and port 25
The Internet uses a mechanism of transporting data called TCP/IP. Built into the framework of TCP/IP are many types of traffic. Some examples are web pages (http and https), FTP, Email retrieval (POP3 is one), and Sending email (SMTP).
SMTP stands for Simple Mail Transfer Protocol. To put it in simple terms, it is the framework designed to run over TCP/IP for the purpose of relaying mail. What is relaying mail? When Usermail.com takes your messages and transfers it to a Gmail.com server, this is considered a relay.
Now, let’s take this a step further. Based on the requirements of TCP/IP, each service needs to use specific ports. There are more than 65,000 ports that TCP/IP uses, but port 25 has been exclusively reserved for SMTP traffic. So, when you send an emailing from Usermail.com to Gmail, our servers will connect to the servers at Gmail.com on port 25 and relay the message.
Hopefully, this explains why, when setting up your email client, the outgoing server uses port 25 as its default port.
In my next article, I will explain why many customers have to use port 2525 in order to relay through Usermail.com.
Brian F.