COMPUTER NETWORKING

Detailed explanation-1: -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 However, this is an outdated implementation and port 587 should be used if possible.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-2: -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.

Detailed explanation-3: -SMTP relaying is the transmittal of email from email server to email server. You have to accept SMTP traffic at port 25 to receive communications from other domains. This is unsecured, information is sent as clear text / ASCII. It is not sent over SSL.

Detailed explanation-4: -What is Port 25? When email is delivered, servers talk to each other using the Transmission Control Protocol (TCP) over port 25. Just as the Hypertext Transfer Protocol (HTTP) that powers the web uses port 80, SMTP uses port 25.