I have fixed this issue in our setup. Same error. Turns out that the Active Directory authentication done by the MX box requires TLS. For that, the chatter uses an SSL certificate. I had just finished replacing our DC with a newer box. Since it was only being used as a DNS server, only the barebones installation was done. As you, I panicked a bit when I saw the errors fill up the event log. After sleuthing around and reading posts everywhere, ya'lls reminded me about the active directory link with the MX box. Checking on it, the console reported the error where it could not authenticate with the new DC. Reading the requirements for this setup, I found that it needed an SSL certificate. I installed the IIS role and created a self-signed cert. Once that was done, the MX was able to connect and when I checked the server's event log, the messages no longer appeared. Hope this helps!
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