Interestingly, mail providers define the connection settings, not the mail client. As Network solution choose to lock non customers out of their support files, I can not even access them to look that information up. AOL has no such corporate problems and freely publish their settings https://help.aol.com/articles/how-do-i-use-other-email-applications-to-send-and-receive-my-aol-mail However they proactively register their settings with the Thunderbird team and when an account is setup in Thunderbird this database is accessed to get the settings. https://autoconfig.thunderbird.net/v1.1/aol.com If you are having issues with both providers at the same time, it is more likely to be a third party product like antivirus that is causing the failures, rather than Thunderbird.