This “fix” is for a NEW installation of TB only, since deleting the “profile.ini” and “xxxxxx.default” files will delete all emails from previously existing accounts you may already have had. If you encounter the “incoming server already exists” issue while trying to add a new account to an existing TB installation containing emails in another working mail account you have had for some time, you must export or otherwise save those emails first. After re-establishing the mail accounts as per my suggested “fix”, the old emails will have to be reimported. Exporting emails, or otherwise saving them, as you probably know, is a whole, ‘nother difficult issue in Thunderbird.
Uninstalling Thunderbird does NOT remove its “Profile.ini” and “xxxxxxx.default” files in their respective directories on the C: drive. Thus, a reinstallation of TB picks up the old undeleted profiles and that’s why you keep having the “incoming server already exists” problem. In XP/2000 navigate to C:\Documents and Settings[User Name]\Application Data\Thunderbird. In Vista and Win7, navigate to C:\users[User Name]\AppData\Roaming\Thunderbird. You will see a subdirectory named “Profiles”, and also the “profiles.ini” file. Delete the “ini” file and go into the Profiles subdirectory. There you will find the “xxxxxxx.default” file, where the x’s are random characters, which you can now delete. (The very useful eHow article on “How to Transfer Thunderbird Settings and Email on a Windows PC” put me on to this idea).
Bring up Thunderbird—you may get a screen asking what program to import information from: be sure to click on the option which says NOT to import anything. When Thunderbird comes up, it will have no account information (since you just erased all of your previous account information, including the fouled up previous attempts that were screwing everything up with that frustrating “incoming server already exists” brick wall), and you will be asked to create an account. Enter the email address, and the password that will get you into your email account on your ISP (and keep “remember password” checked). IMPORTANT! When you click on the continue or OK (whatever) button, click on “stop” in the next screen as fast as you can, or wait until TB tells you that it has found your (albeit incorrect) account info for your ISP server. Click on “edit” and start entering the CORRECT information, including POP instead of IMAP, if necessary, and the correct port information. If the outgoing server is wrong, just leave it for now for later correction.
VERY IMPORTANT DO NOT hit the “Retest” configuration (or whatever) button, and DO NOT hit the “Create Account” button on the lower right. HIT THE “Manual create” BUTTON at the LOWER LEFT. You should see your new account appear in the left hand TB column. Now open the “Tools” menu in TB and select “Account Settings”. The bottom entry that appears in the left hand column is “Outgoing Server (SMPT)”. If it is incorrect, click on it, and on the next screen that appears, click on “Add”. Enter the correct information for your server’s outgoing mail and click “OK”. Go back into “Tools”|”Account Settings”, and in the left hand column, click on the email address for the account you have just set up. At the bottom of the screen that appears there will be a pull down menu next to “Outgoing Server (SMPT)” where you will find, and can select, the correct “Outgoing Server (SMPT)” you have just set up.
For each email account you want to set up, go back into “Tools”|”Account Settings”, and underneath the left hand column, click on “Account Actions”, and select “Add Mail Account”. Repeat the above procedure from here for the new email account.
Deleting the “profile.ini” and “xxxxxxx.default” files from the Thunderbird “App Data”, or “Application Data” files finally allowed me a totally clean setup of my email accounts as described above. Hope this helps the many other people I came across on the internet at large who have been enduring endless, fruitless frustration over this issue.