Removing an account from Thunderbird will not delete any emails.
Access 'Account Settings' by selecting the icon with 3 parallel bars on the far right of your Thunderbird window, select 'Options' and 'Account Settings'.
In the 'Account Settings' window, remove an account by:
1. Select the account you wish to remove from Thunderbird
2. Select the 'Account Actions' drop-down menu towards the bottom of the menu
3. Select 'Remove Account'
4. Select 'Outgoing Server (SMTP)' on the left side of the 'Account Settings' window
6. Remove the old outgoing server by selecting it and clicking the 'Remove' button on the right side