SMTP by default uses TCP port 25. The protocol for mail submission is the same, but uses port 587. SMTP connections secured by SSL, known as SMTPS, default to port 465 (nonstandard, but sometimes used for legacy reasons).
Simply put, computer ports are the communication endpoints of a computer connected to a network. They are separated to differentiate the several processes and make it easier to handle them: so each one has a particular purpose and is associated to a specific protocol.
Usually an outgoing server employs port 25: it’s the default SMTP port. However, some IPs deny its use because of the massive spam and malware traffic by which is affected. This issue is raised in particular when you need to switch to another ISP – for instance, when you’re travelling and connecting to a new provider. In this case you can try to use port 587 or port 465 to avoid the block.
port 587 is supported by almost every outgoing SMTP server and it’s useful for unencrypted or TLS connections; while port 465 is the right choice if you need to connect via SSL.
If you want to know how to change SMTP port on Microsoft Outlook, please see the article below :