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Not sure specifically but I'm sure the SE you're working with or support under a temporary grant can do a webex and figure out what config issue you're battling. They do after all have a vested interest in making your POC successful. The "sending email" portion of the email configuration can be confusing. Is it possible your ironport is rejecting the mail since it's coming from an IP that in the past hasn't normally relayed mail through it?
The way I did the POC and tested MEG without disturbing the client's production mail was to take an unused domain of the client and points its MX record to the MEG. For outbound, an Exchange connector was defined to use the MEG as the outbound hop for a specific external email address domain (I used my personal domain... such that from Outlook if any mail was going to be sent to *@mypersonaldomain.com, Exchange would route it out through the MEG).