I tested Legible Mail (link in signature) and it seemed to work fine. Here's a snip of the fontObject:SetFont() call.
Lua Code:
local fontObject = CreateFont("LegibleMail_Font")
local isOK = fontObject:SetFont(LSM:Fetch("font", sdg.font), sdg.fontSize, "")
OpenMailBodyText:SetFontObject(fontObject) -- READING
SendMailBodyEditBox:SetFontObject(fontObject) -- WRITING