You should be able to right click and get spelling suggestions though.
As of Beta 3 / build 1278 we now provide corrections via Prism xulrunner dictionary (it previously just underlined):
Bug 30610 - Spellcheck suggests don't work in email editor
In RC2 / build 1513 since adding Aspell would require Apache, we hid the button to avoid confusion:
Bug 23112 - support spell check
(Most modern browsers also have spell check built-in should you be using another with Zimbra Desktop. We may throw in a light web container later, or possibly someday run Aspell with the existing Jetty, but adding another server product when there was already Mozilla's integrated checking seemed a waste of system resources and effort at the time. Especially since our Aspell dosen't yet have multiple language dictionaries.
Bug 22255 - Spell checker not i18n ready)