FUNDAMENTALS OF COMPUTER

Detailed explanation-1: -Port 25 is the default port for relaying email on the internet. However, you can specify any port number (Provided it is within the acceptable range of custom port numbers) for submitting emails to your SMTP server. There are special cases when you must use port 25 on your SMTP server.

Detailed explanation-2: -Originally, the Simple Mail Transfer Protocol (SMTP) used port 25. Today, SMTP should instead use port 587-this is the port for encrypted email transmissions using SMTP Secure (SMTPS). Port 465 is also used sometimes for SMTPS. However, this is an outdated implementation and port 587 should be used if possible.

Detailed explanation-3: -Tl; dr Use port 587 if you can, 465 if you can’t, and 25 if you must. Port 587 is technically correct, the best kind of correct. However, many ESPs have adopted implicit TLS on port 465. While you can send email over port 25 and 2525, it’s much more secure to have the messages encrypted.

Detailed explanation-4: -The SMTP transmission channel is a TCP connection established between the sender process port U and the receiver process port L. This connection is used as the transmission channel. This protocol is assigned the port 25, which is L=25 as the default transmission channel for communication between mail servers.

Detailed explanation-5: -Sending emails over port 25 is not really secure and because this is the default port, so it been extensively used by the spammers too.