We have 2 OU's in AD, one titled Internal ONLY DL's and the other Internal/External DL's. I noticed that they are controlling whether an external sender can send to the DL's depending which OU it's in, regardless of the Exchange settings on the DL's.
There's no direct GPO linked to these OU's, only the default. I can't find out how these OU's are controlling the DL's. I looked for 3rd party software on the server but there was none.