code: @Nullable
    public FailedMailData remove (@Nullable final String sID)
    {
      if (StringHelper.hasNoText (sID))
        return null;
  
      return m_aRWLock.writeLocked ( () -> internalRemove (sID));
    }
docs: Remove the failed mail at the given index.
  
  @param sID
  The ID of the failed mail data to be removed.
  @return <code>null</code> if no such data exists