If you are using the new wysiwyg editor for newsletter, you can enable the spell check by clicking "Spell Check As You Type" then select "Enable SCAYT".
Once the Spell Check is enabled, you can use it by right clicking on the words with the red curly line. It will display a list of suggested words for you to correct.
You might wonder why we don't default it as "enabled". Well, this is completely for the performance reason. If we enable it by default, it would take longer to load your page. We think it's best for you to be able to control it.