[mailop] Cryptic Earthlink rejection message

[Russell Clemings via mailop]

    host mx03.oxsus-vadesecure.net [147.135.97.26]
    SMTP error from remote mail server after end of data:
    550 5.7.1 Message rejected - OXSUS0001_507

Anyone know what this means? (Besides the obvious.) Googling
"OXSUS0001_507" yields nothing. It's coming from mail sent to Earthlink.net
and related domains: jps.net, mindspring.com, netcom.com so far. Just

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