I've been a happy Claws Mail user for over a decade, but recently the lack of HTML email composing has just become irksome (primarily because of the RFC822 imposed line breaks that are required in plain text email).
But I've tried Sylpheed and Trojita, and neither of them has HTML composing. Geary seems too simplistic. Is there a lightweight client (i.e. one other than Thunderbird) that is at least reasonably customisable?