We have written a routing agent for our transport service which modifies email attachments destined for external recipients. Our problem is that sometimes internal recipients who are copied on these emails do not receive the modified email. We're running Exchange 2013 with 3 servers (CAS+MBX).
I believe this is by design, i.e. how the mail flow / mailbox transport service works.
Question: How can we force all emails to go via the local transport service, such that both internal and external recipients will receive the modified email?