Square's SMTP Server without authentication

Few months back when Square started the bug bounty in Hackerone
After some dorking I found this "api-test.squareup.com"

Yea a test bed.
Usually these test sub-domains are vulnerable because they not maintained after initial development phase.

Visited the sub-domain alas nothing to see.
It was a blank page.

But the world doesn't end here.
Here comes the Nmap.

Scanned and found port 25 was open.

And hopefully it has no authentication
so "telnet api-test.squareup.com 25"

So, I can  send mail using the Square's Server to anyone.

I reported it they fixed it within few week.