I'm running exchange 2013 and I'm trying to restrict DL's to only receive email from "only senders inside my organization".
I found the option, it's checked, but it doesn't work, as I have some emails failing wth the error "550. 5.7.1 Resolvever.rst"
That's because I had to turn off the option "externally secured" on one of my receive connectors.
If I enable the "externally secured', then anyone can send to the Distribution Lists again even though the option to "only senders inside my organization" is checked.
I couldn't find a fix for this online, so perhaps I missed something?