Find "Mail"
Press the menu icon.
Press Mail.
Select email account
Press the Menu key.
Press Accounts list.
Press the required email account.
Select default email account
Press the Menu key.
Press More.
Press Settings.
Press General settings.
Press Set as default account to turn on the function.
When the box next to the menu item is ticked (V), the function is turned on.
Exit
Press the Home key to return to standby mode.