TinyMCE is the WYSIWYG editor we use. It does not support nested tables which are common in email design. The table can be created using the following steps:
  1. Click outside the current table in the message
  2. Click Insert>Table
  3. Select the column/rows and insert it - this creates the table outside
  4. Highlight the new table you created and click ctrl X to cut it
  5. Click your cursor where you want the new table and click ctrl V to paste
  6. Resize as needed, then drag/drop your content
  7. From here, modify the html code to remove the dashed border

Video tutorial