GtkSpell provides word-processor-style highlighting and replacement of misspelled words in a GtkTextView widget. 
Right-clicking a misspelled word pops up a menu of suggested replacements.

GtkSpell depends on
    *  GTK+ 2.0 or greater.
    * Either Kevin Atkinson's pspell library, or its successor, GNU Aspell.
GTKSPELL IS requiered by sylpheed for spell checking function to work