I suggest you contact Spectrum. This is not an issue for an email client. By contacting them, you can get explicit details on the issue.

I have contacted Spectrum... they could not find any error code anywhere near what was shown. I could enter https://webmail.spectrum.net/mail/auth and sens anything anywhere with or without attachments. I don't usually send emails so I do not know when it started. now I have w2ished to send three in the last week or so... I can't... I have had Thunderbird for years now.... I dont want to .... yet I will find an email server that will let me send. and it will not be spectrum.... any suggestions? tom