Comment by whizzter

The difference in ports is due to SSL/TLS expected to be automatically applied on 465 iirc like 80/443 for HTTP(S) (you can also encrypt 25 by issuing STARTTLS after setting up a connection but it's not the default and might fail I think).

Authentication with mail is separate, usually to allow for relaying whilst anyone can usually drop emails IFF your server is the destination.

Confusing and needlessly complex? Yep. Natural result as of uncontrolled evolution? Yep.