I too have had the same message. I set up a subscription that works when I send to a list of VALID email addresses specified in the format:
firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
I get an error message indicating there is an invalid email address when...
1. the list contains an invalid email address (i.e., when an employee leaves the company)
2. I use an Outlook Group name, which is what I wanted to do originally, instead of a list of of email addresses. Obviously, it's easier to manage the receipient list for several subscriptions that all sent to a group name instead of having to modify a long list of email addresses in several subscriptions. I would specify the Outlook group name by supplying our email suffix, as in: EmailGroupName@xyz.com. But that doesn't work either.
Anyone have any suggestions for a better way to manage email addresses.