THe problem may be tne new account setup wizard. try this: click to settings>general and UNtick the experimental setup option and retry the setup. see attached.

Yep, tried that still won't connect

thx

The error "incoming server already exists" indicates the account is already in the account setting pane when you open it and you are trying to add it a second time. As for the other items specifics not general assumption are required to do more than guess as David has done.

For instance. What mail server are you trying to connect to? The Thunderbird project maintains a database of mail providers where users exceed around 100,000 accounts so big providers like Gmail, Yahoo, Outlook and even some ISP's like ATT and free.fr are included. Not knowing if your account should be in that database really leaves everyone guessing.

 In this case the data was located in the database.  Specifically here https://autoconfig.thunderbird.net/v1.1/google.com

However if I change the email to willaWonka@WonkaChocolates.com Thunderbird fails entirely because the domain actually does not exist. The screen does eventually get to that nugget of information under the Hostname dialog when it says hostname empty or invalid. A look at my error console shows

08:49:22.702 The config file XML does not contain an email account configuration. 

THis mean no enbtry in the database. The log then attempts to connect to the domain using an assortment of common mail domains for that adress, adding SMTP, IMAP and POP top the start of the domain name and attempting to connect using a selection of ports, however this all falls in a heap because the log reports.

08:49:36.374 mail.setup: guessConfig failed: Can't find a server email.mjs:967:29

So we know for certain that the server does not respond on any of the 24 names or ports attempted. It truly is guessing.

What do you see in your error console? (Ctrl+Shift+J)

I was having an issue adding a yahoo.co.uk mail account and David's advice above to turn off the new experimental feature worked. This should not be enabled by default on new Thunderbird installs. The goal of Thunderbird stable should be to provide the most reliable and stable experience possible with the latest build, turning on experimental features as default seems to be a silly idea. Just my personal opinion. Thanks David.

I fully agree. But I have this feeling it won't happen. :)