We are users of Office 365 and they handle our exchange server. This works great.
Recently, email addressed to certain companies (e.g., asplundh.com) are bouncing. From the headers it seems that the mail never makes it to Asplundh.com. The message seems to stop with a Microsoft SMTP server. Neither my domain nor the destination domains are blacklisted anywhere! Also, exchange spam checking is reporting a spam-free email.
However, the Delivery Status Notification (DSN) says the remote server, mx16.ess.sfj.cudaops.com, is reporting an error.
Does anyone understand why cudaops is even being conferred? Is this part of a Microsoft process?
Original Message Details
1/10/2019 7:50:58 PM
Expression of Interest
550 5.7.350 Remote server returned message detected as spam -> 550 permanent failure for one or more recipients (mailto:firstname.lastname@example.org:blocked)
DSN generated by:
Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384)