https://www.blackhillsinfosec.com/how-to-test-for-open-mail-relays/
Becoming less common, but still out there.
- misconfiguration in the mail server allowing us to perform Mail Relay
- Now the more common is partially open
- Send email from an external source address to an internal destination address
- Send email from internal source address to an internal destination address
- Quite useful for phishing attacks