The port mail servers use to hand messages to one another. When a message leaves your provider for the recipient's, it travels over port 25, and that has been true since the early 1980s. It is deliberately not the port your own applications should use to send mail — that is 587 — because 25 carries no expectation of authentication.
A server that relays mail for anyone is an open relay and will be found and abused within hours. Most residential ISPs and cloud providers block outbound 25 by default specifically to limit spam.
Worth knowing. Do not use 25 for your own applications sending mail. Use 587 with authentication, which is what it exists for.
An open 25 on a mail server is correct. On anything else it is usually a forgotten default, or a compromised host relaying spam. Test that it is not an open relay, and check whether your address has landed on a blacklist.
The port scanner will tell you whether 25 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.