I am experiencing the same symptoms using Outlook on our home PCs running on our home Cox network. Same issue occurred around June of 2019, which resolved as mysteriously as it appeared. Now it is back.
Issue: When running Outlook mail client on either of our home PCs -- Outlook 2013/Windows 8.1/POP3, Outlook 365/Windows 10/IMAP -- I am unable to send email from any of our cox.net addresses to any other cox.net address, including our neighbors who happen to have cox addresses. Self-to-self mails show up in the Outlook 365 SentMail folder AND in the Cox Webmail "Sent objects" folder, but never make it to the Inbox in either Webmail or Outlook. No error messages. Traffic from Cox to other addresses -- outlook.com, gmail.com -- works fine, both directions. Traffic originated in Cox Webmail works fine. All PCs are up to date, with latest Outlook builds, and Cox-recommended POP3 and IMAP settings.
I am however able to send cox-to-cox emails when:
I connect my PC to my TMobile LTE network via hotspot, or I move my PC to another Cox home network (Chandler, fyi. My home is in Phoenix.)
I use my Android Outlook client on my smartphone (connected to the LTE network or to my Cox home network via WiFi), or my wife uses her default iPhone mail app.
Any suggestions?
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.
