Interesting - I can't find any option for spell checking and yet one of the "suggests" dependancies on the Debian package is the apsell spell checker. Strange.

IIRC (I might not!) Sylpheed doesn't have this option, but the "bleeding edge" version sylpheed-claws does. (apt-get install sylpheed-claws)

Check the sylpheed homepage, it'll probably have more accurate info.

Ricardo