Automator: How to send an email to multiple recipients with Font colour and attachment image?
As far as I can tell, there is no direct way. You would have to use applescript to select the text and set the formatting in the Mail application after the fact... (Example below taken from the web...)

tell application "System Events" to tell (process 1 where frontmost is true)
    set {c1, c2} to value of attribute "AXSelectedTextRange" of text area 1 of scroll area 1 of window 1
end tell
tell application (path to frontmost application as text) to tell document 1
    tell characters c1 thru c2
        set font to "Menlo"
        set size to 18
    end tell
end tell