Having Issues with Optimum mail

I can receive but cannot send mail using Apple mail. I can send and receive if I go onto Optimum webmail page. Started yesterday and it is still occurring. For a while when on cellular network i could send but now that doesn’t work either. I’m sure it is at the Optimum end but they couldn’t figure it out. They put in a request for their tech. Department to investigate. I’ve deleted all accounts and renter them.

still can’t send. All devices in my home are having the same problem Three iPads, two iPhone and two Mac. Any ideas? I thought maybe the router was the issue so bypassed it. Problem still occurs.