What port should you chose for your SMTP server when you’re setting it on a mail client?
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.