To follow up on Poesjkin's message, that error text is exactly what an IronPort ESA will give when it applies rate limiting. If the receiving MTA is indeed an IronPort ESA, then that drastically narrows the likely reasons for the rate limiting. The most likely reason is that your sending ESA has an IP address with a poor SenderBase reputation, or has no reputation at all. Another good possibility is that that IP address fails the address->name->address double-lookup DNS verification. Try investigating those and see what you come up with.
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We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...