As far as I can see, there is no way to manually spell check a message or to have it checked "on send" in Zimbra Desktop 7. This function seems to be available in the web interface, so it would be nice to see it in ZD. This is a barrier to adoption for some of our execs who feel that their workflow is hindered by the realtime underlining of misspelled words (like "realtime" and "Zimbra"). ("It slows me down to have to worry about correcting my spelling while I'm writing.") I think it's being a bit... silly... but these are the people who work with me on deciding whether or not money gets spent on a product. My vote is an important one, but I am outnumbered on this issue.
If I'm missing something, please let me know. If not, please consider adding the same functionality that already exists in the web client to ZD. It would make our GM smile.