We are using exchange 2010 and recently implemented the policy where people can't relay through our exchange server without either authenticating or we allow their IP. When we did this, when employees send to a distribution list on another organization's mail server but includes email addresses with our domain in them, the message doesn't get delivered and the 550 5.7.1 message comes up. I'll try to show an example:
We are domain A and we have employees that sit in different organizations. Two specific ones are showing problems right now, we'll call them domain B and domain C.
When Joe from domain A sends an email to the distribution list with the email address firstname.lastname@example.org (which includes email addresses from domain A) those people in Domain A do not get it. Same happens to those who send to domainc.com. This JUST started happening this week and it ONLY seems to be happening when users send to distribution lists. If a user within the different organizations (domain B and domain C) send, it goes out just fine.
How can I tweak it so our employees who use our email can send to these lists and have our other employees get them? Thank you.