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It is some middleman inspection, yes.
Just because some SSL certificate is used doesn't mean the traffic can't be inspected.
FortiGate Firewalls amongst other have those type of inspections where the SSL certificate is attached to the Firewall posing as the Handshake authority, so the SSL connection is set up with the firewall, the firewall then inspects the traffic and passes the ok'd traffic through to the mailserver.
This is man in the middle, but in this sense there's nothing wrong with it in my opinion. I used the Barracuda Spam Filter in the same way.
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