port 25 but now you're telnetting port 587

Generally, mail clients should submit to port 587. Port 25 is more and more restricted to communication between mail servers. See also https://www.mailgun.com/blog/which-smtp-port-understanding-ports-25-465-587 (first Google result).

Of course, as long as you work in an isolated lab environment, you are free to use port 25 (or, in fact, any port you like). But when it comes to using real world mail servers, use port 587.

Alexander