Lol, today i have send me a new Mail from within AquaMail and i have Arial 10 in the Signature... I dont know, where or why this is changed and what i have done, but i ended up with the following code:
<span style="font-family: Arial, Myriad, 'Liberation Sans', sans-serif;">
I have no glue, what i have done, but this is what i want (or better a working workaround). Its Ok for me with this signature...