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We noticed that by default "Require SMTP Authentication" is disabled for a domain. I did some test sending with SMTP Auth disabled and SMTP Auth enabled for a domain and did not notice a difference. With SMTP Auth enabled or disabled for a domain:
1. SMTP Auth is required to relay
2. I can send from a local mailbox to another local mailbox with no auth
Our server also has the default "Nobody" for allow relay and "Allow relay for authenticated users" checked.
With a server with the default settings, does enabling "Require SMTP Authentication" for a domain make a difference?