Sometimes people report this and later find that they didn't delete the account properly. It may be useful if you post the steps you took in deleting the account. Thank you.

Thank you for ur answer. I open Thunderbird from Windows Start. Then I open "Extras" in the menu bar. In English it's called Tools or something. Then account settings. Then I select the email account. Then I select Account Actions and Remove Account

And now the Account is removed. But when I close Thunderbird and reopen it, the account is appears again.

Okay, just a guess here... If you have other accounts, check to see if one of them was using a component of the deleted account. For example, it sometimes happens that a person is using two accounts for the same smtp server. My suggestion that 'may' verify this is to -highlight the account, click the account actions and remove account button as before. - now, click help>moretroubleshootinginformation and scroll down to where it displays a matrix for MAIL AND NEWS ACCOUNTS. See if any component of the deleted account appears there. - another test is to click settings>privacy&security, click 'saved passwords'. That will cause a matrix to display, showing every server in the profile. This listing should be a multiple of two. If not, that may be a hint on the problem. Look to see if any component of the deleted account appears.

Thank you very much. I understood and executed the second part. But it didn't do anything. The account will be back on reboot. The first part of your answer gets me to the part about deleting the account. After that, however, there is no way for help. Isn't it possible that TB has a mechanism that protects e-mail accounts twice and that you may have to delete something in the profile folder or even to edit the file prefs.js or something else?

No, that feature does not exist. What happened when you clicked to help>Moretroubleshootinginformation and again when you clicked 'saved passwords?' Did you see any remnants of the account?