Create Message with Apple Mail from commandline
I haven't really tested this, but you could use AppleScript for it.

mymail() {
    osascript - "$@" &lt;&lt;-END
on run args
tell app "Mail"
set m to make new outgoing message with properties {subject:item 2 of args, content:item 3 of args, visible:true}
tell m
make new to recipient at end of to recipients with properties {address:item 1 of args}
if number of args is 4
make new attachment with properties {file name:POSIX file (item 4 of args)} at after last paragraph
end
end
activate
end
end
END
}