Many email accounts can be created within this email client and this makes it easy to see all emails from one screen. Emails can be composed by opening the Thunderbird email client or via links that user clicks to activate the email composer. In order improve efficiency, one of the accounts can be assigned as the default email account and this is normally indicated in bold. This allows all new emails to use this account and users do not need to choose the account. I have listed the steps tested for Thunderbird version 24 and 31 where the example shows account2 in bold as the default account and we want to switch the default to account1.
Step 1: Open Thunderbird account's setting.Start Thunderbird and from its menu bar at the top (default), choose Tools ->Account Settings...
Identify the default account that is used to send out emails. This is in bold, for example "account2"
Step 2: Set the default accountChoose the account to be used as the default. In this case, its "account1". At bottom of the window, click Account Actions. Choose "Set as Default".
Click OK, notice that this email account will go to the top of the list and is in bold font. Ensure all email screens are closed and restart Thunderbird.