Your provider only allows email clients to send mail using their servers if it connected to the internet using their service. This restriction goes back more than 10 years to the hayday of Comcast and Charter and their plans to lock users into their service for phone and desktop. Spectrum I assume have seen no reason to change the policy. You might be able to get a VPN to drop you into an IP address supported by SPectrum, but I do not like your chances. They do not place this restriction on the webmail website, only email clients that use their SMTP server to submit mail. Note that the service level agreement included in your spectrum service for email only requires them to make a email service available. So they save the dollars on proper user support by simply supporting your ability to log into a website, if you can their obligation is met. Lower support costs for them and the need to hire competent email engineers is lower as email is technically offered, but in reality totally unsupported.