Mail submission where TLS is established immediately, before any SMTP conversation. Deprecated for years, then formally reinstated as a valid submission port.
Encrypted from the first byte, which makes it harder to strip than STARTTLS. A reasonable choice alongside 587.
Open 465 on a mail server is fine, and arguably preferable to 587 since TLS is established before anything else is negotiated.
The port scanner will tell you whether 465 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.