Thanks for the prompt reply, Allan. Note I am also Middlejimbo...had to create a new name to re-join Forum for some reason.
I am not using a VPN. I get why you are suggesting a router bypass, and I will perform that experiment this evening, but a router filter doesn't explain why cox-to-cox mail are showing up in my Cox WebMail "Sent items" folder if they were not making it past my home network? You are free to dig around in my WebMail to analyze those sent-but-never-received items.
UPDATE: In an attempt to avoid sending cox-to-cox for a mass email (~100 addresses, which includes multiple cox.net recipients), I originated the email from my outlook.com address. I received an Undeliverable notice listing all of the cox.net addresses indicating timeout due to "450 4.4.318 Connection was closed abruptly (SuspiciousRemoteServerError)". When I sent the exact same email to a single cox.net addressee from my outlook.com address, and it went through without error.
