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We appreciate you being a loyal customer of over 30 years and don't want you to feel this way about your email transition experience. I rely on my email too for access to important messages and ducoments All emails and folders in your inbox on Cox webmail would have transitioned to the Yahoo mail servers. If you have messages that you downloaded to your device(s) prior to the transition they will still be on those devices.
Have you already signed in to the Yahoo mail page to complete the transition with your cox.net email address and password and accessed your inbox?
As a part of the move process, the various settings that allow a third-party email client to operate will need to be created on your part to reflect the new Yahoo settings if not using the Yahoo Mail app or direct website. See https://help.yahoo.com/kb/fix-pop-imap-client-sending-receiving-issues-sln3792.html for more information.
Please visit https://www.cox.com/residential/support/access-and-log-in-to-yahoo-mail-after-transition.html for more info about the ways you can access mail after the transition. Once you have successfully completed the mail transition, Yahoo provides all support. You can visit help.yahoo.com or call Yahoo Support at 1-866-562-7250.