I'm trying to understand how SMTP servers work in production environments. I am wondering how email can be sent from server A to server B if server B requires a login.
A real life example of this would be when someone sends mail from Outlook to Gmail. Based on my current understanding, Outlook would need to authenticate with Gmail's servers. Is there a port, or a protocol that allows an email to be sent from one server to another without authentication? I haven't been able to find any articles or information that describe this.