I'm a bit puzzled at how this is happening:
We have an external customer who is sending email FROM their email address, On Behalf of one of our internal email addresses. So, For example, let's say "Nick" is the external customer, "Todd" is internal employee, and "Greg" is an internal employee. Nick@external.com sent an email to Greg@internal.com On Behalf Of Todd@internal.com.
How in the world can Nick (Being an external customer) send email to a 3rd person, on behalf of one of our internal users???
I double checked the security tab in AD for our internal users and nothing seems out of place. We can only select from our GAL in Outlook for delegation, so I don't think the user was able to grant Nick send on Behalf permission.