Optimum mail is not working when using wiFi. Connection to outgoing server failed. Sender Email. invalid
My optimum mail does not allow me to send out mail while using the Apple Mail app and WiFi. If I turn WiFi off it and use cellular it works but if I turn wifi on I get the Sender Email invalid and connection to outgoing server failed message. I can receive emails but can’t send. If I use their web browser there are no issues. I bypassed the router but the issue still occurs. It occurs on all my devices, two Macs, three iPads and two iPhone. The iPads and iphones are running iOS 14.3. One of the Mac’s use Yosemite and the other uses Mojave. The 2014 Mac is attached by Ethernet and it behaves just like the others. It fails to send. Any ideas? Could it be the cable modem?
Mac mini, OS X 10.11