When I use power shell to Send-MailMessage from our domain user's email address to any external email address, then I receive "5.7.1 Unable to relay" which is as expected.
Unfortunately, when I Send-MailMessage from domain user's email address to another domain user's email address or the same domain user's email address, the message is sent without asking for username or password.
Please help me to prevent sending any email without authentication in Exchange Server 2013.
