iCloud Mail and Comcast fail to send mail

Hi gang. I cannot send email in Apple Mail, Sonoma 14.4.1. My WiFi ISP is Comcast. My Google Mail's POP server account sends fine but every time I try to send via my iCloud account I get this error message. It looks like Comcast is trying to send via one of their SMTP servers and not iCloud POP server. I have to use my iPad, which is also on Comcast WiFi, to send mail. Would anyone advise?