Use your favourite registry editor and do the following to retrieve back the default mailbox.

Use Email Composer
[HKCU\Software\Microsoft\Inbox\MsgTypes\IPM\Note\]
Change the value of DLL from EmForm.dll to htmlnote.dll

Use SMS Composer
[HKCU\Software\Microsoft\Inbox\MsgTypes\IPM\SMStext]
Change the value of DLL from mms.dll to htmlnote.dll

Enable Threaded SMS
[HKCU\Software\Microsoft\Inbox\Settings\OEM\]
Change the value of SMSInboxThreadingDisabled from 1 to 0

Enable auto-complete for To: field (only available for default Keyboard)
[HKLM\System\CurrentControlSet\Control\Layouts\e0010409]
Change the value of Ime File from \windows\XT9ime.dll to \windows\compime.dll
Change the value of Layout Text from "Samsung Keypad" to "COMP IME"
(excluding the quotes)