The steps vary a bit from device to device, but the following steps work on older Nokia S60 devices (as Nokia E71, X6-00, 5230 and similar) in which has not have the built-in Nokia Mail Client:

  1. On the device, select the menu key.
  2. Select Installations.
  3. Select Email.
  4. Highlight the mailbox that needs to be removed.
  5. Select Options.
  6. Select Remove mailbox.
  7. Select Yes.

Reboot device by removing battery after mailbox has been removed.