Not sure if this effects you, but this was info off the spectrum website: ''Note: Spectrum Business customers with an email address ending in @bresnan.net should refer to a recent email notification from us detailing important changes, including how to save email and contacts in advance of the changes occurring on March 6, 2024. Questions? Call (800) 314-7195.'' The part which comes after the '@' in your email address determines which server settings you should be using. It would be helpful if you could tell us what comes after the '@'. Also provide info on current server settings: In Thunderbird * Help > Troubleshooting Information * Scroll to 'Mail and News Accounts' section. * Copy all the data in that section and paste into this forum question. Useful link: https://www.spectrum.net/support/internet/spectrum-email-server-settings