Where an email client or application submits outgoing mail to its own server, with authentication. Distinct from port 25, which is for server-to-server delivery.
Should always require authentication and STARTTLS. This is the port your application should use to send mail, not 25.
Open 587 is correct for a server accepting mail submissions. Verify it requires both authentication and STARTTLS, and that it will not relay for unauthenticated senders.
The port scanner will tell you whether 587 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.