Have been using mail/same email address/cox.net provider for long time. Success in sending emails is not good. I get message that cannot send SMTP mail on my SMTP.COX.NET server. Then after many minutes it is sent. Any suggestions?

Please confirm that you are running the very old Panther OS 10.3

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Contact COX to see if there is a problem with their server.

Mail troubleshooting - Yosemite

Mail (Yosemite): If you can’t send messages

I have just had a similar problem today. I have two MacBook Pros (running 10.10.5 and 10.10.1) and an iPhone with the same five email accounts on the two computers and one on the phone. The email authentication stopped working on all of them. Once I circumvented the Airport express and connected directly to the Comcast modem, I was able to send and receive email again. I can use everything else on the internet just not email. The phone using cellular data can get mail just fine. Put it back on the Wi-Fi, nothing.

I tried different cables, reset the Airport to factory defaults, same results. It seems like the Airport is not forwarding the traffic required for IMAP authentication. I might downgrade the firmware next, but after that, I think it is replacement time.

I checked with my email provider and there are no problems there. They helped me verify settings etc. Webmail _does_ work through the Airport.