Re: [client] did not issue MAIL/EXPN/VRFY/ETRN during connection

After removing all the timeout settings and the "Kcheck_client dns" option, nothing changed - except it now fails after 5 minutes instead of 1 minute with the exact symptoms.
I found some directions at Proofpoint site about disabling Cisco's SMTP fixup options, and confirmed no such options are configured/available on these devices sitting in front of the MX servers. There is no decryption/deep packet inspection happening for these connections. The IPS/IDS systems are not dropping any packets of the sessions.
And confirmed all name resolution is performed in under 2 seconds - during the monitored sessions, successfully resolved immediately before the new connection entry was recorded by the mail log file.
On a positive note, I got information from the remote end:
Remote server returned "554 5.4.0 < #4.4.2>"
Unless I'm reading this wrong, 5.4.0 seems to be on the remote/sender side, so not much I can do on my end.