A “port” is another way to help computers ( like two mail servers) communicate with one another: An IP address identifies a computer. A port identifies a specific application/service running on that computer, like SMTP.
More in detail: 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. turboSMTP can work both on port 587 and 465, but also on 2525 and 25025. Moreover, it completely avoids the “another ISP” issue: in fact, once you have set our SMTP as default you don’t need to configure it again or switch to other ports.
Port 25 is the original standard SMTP port. Today, the Internet Assigned Numbers Authority (IANA), the group responsible for maintaining the internet addressing scheme, still recognizes port 25 as the standard, default SMTP port. SMTP was designated to use port 25 in IETF Request For Comments (RFC) 821.
Port 25: SMTP port 25 continues to be used primarily for SMTP relaying. SMTP relaying is the transmission of email from email server to email server. In most cases, modern SMTP email clients (Microsoft Outlook, Mail, Thunderbird, etc.) shouldn't use this port.
Port 25 is the oldest of the four. It was the port number assigned to SMTP when the protocol was first introduced in the now obsolete RFC 821 back in 1982, about 33 years ago. In spite its age and the arrival of the other port numbers, port 25 is still very much widely used.
Port: 993. Encryption: SSL/TLS. Server: outlook.office365.com. Port: 995. Encryption: SSL/TLS. Server: smtp.office365.com. Port: 587. Encryption: STARTTLS. MSN. Server: imap-mail.outlook.com. Port: 993. Encryption: SSL/TLS. Server: pop-mail.outlook.com. Port: 995. Encryption: SSL/TLS. Server: smtp-mail.outlook.com. Port: 587. Encryption: STARTTLS
There was a Request for Comments (RFC) 821 was published, to add port 25 as the default gateway to send and receive emails. It has been 3 decades and we are still using port 25 as the default port for transmitting email messages. Over the years, few more RFCs came which actually obsoleted the initial SMTP RFC 821.
A relay server initiates a TCP connection to the server on the "well-known port" for SMTP: port 25, or for connecting to an MSA, port 587. The main difference between an MTA and an MSA is that connecting to an MSA requires SMTP Authentication. SMTP vs mail retrieval. SMTP is a delivery protocol only.