I have a Exchange 2013 Multi Tenancy farm with a Big Mailbox Server hosting all the Mailbox Databases, each client has their own Mailbox Database on the Mailbox Server. I have a Send Connector which transports all the e-mail out via a McAfee Smart Host. The client domains are configured within the McAfee portal and a trusted ip is setup on the portal to allow my hub transport to mail too it. Ok so far so good, this works fine.
If another client (mailbox Database) doesn't want to use McAfee for it's anti-Spam and wants to send via a different Smart Host for example Mimecast, the only way I can see is to add another Smart Host on the same Send Connector. I understand that if you send e-mails through a Send Connector which has multiple Smart Hosts it chooses a different Smart Host each time to load balance them. What I don't understand is what is happening behind the scenes if one of my clients from the McAfee account sends an e-mail out of my organisation and it chooses the Mimecast Smart Host or visa versa. Is the e-mail just getting bounced back or is it failing a handshake and then choosing the other Smart Host? is this going to cause me problems?
Should I be using a third party tool which allows me to specify which domain inside my organisation can send out via different Smart Hosts?