An outgoing mail server works through a particular port to send and deliver emails, where a port is the communication endpoint of a network-connected machine. When you set up an email application, it’s necessary to specify which port your server will use.
The default port of an outgoing mail server is 25: since it “speak” the SMTP protocol, it is also known as the more typical SMTP port. But it’s not the only one, and not even the most reliable for email marketing.
In fact, due to the ever-increasing rate of spam and malware that circulates through port 25, some internet providers and IPs may reject the messages coming from it. There will be no problem whatsoever if you’re sending single emails, but if you’re planning to mail out a bulk message, you could try to use port 587 or port 465 to avoid the block and make your outgoing mail server work correctly.
As explained below, port 587 is suitable for non-encrypted connections, while port 465 is the right one if you connect via SSL: