People seem to prefer avoiding using galvanized parts when possible. Certainly by the time the smoke leaves the mailbox, it shouldn't be very hot, and would therefore be unlikely to cause a problem.
But if something unexpected happened, like the pellets started burning, instead of smoldering, it's possible the temperatures could climb enough to make the zinc on the galvanized elbow become a health risk.
I just ordered a toolbox, to try and do a "mailbox" mod. The toolbox isn't galvanized, unlike most steel mailboxes. I'm going to try and find aluminum duct components if I can, to avoid using galvanized parts.
With that said, I'm sure plenty of people use galvanized duct components, and it's probably fine. I'd just feel better avoiding it.
Not sure what size duct you're using, but
Amazon has 3" 90-degree aluminum elbows, including this one for $7 shipped with Prime:
I had an easier time finding 4" 90-degree aluminum elbows on HD and Lowe's sites. But Home Depot would ship this 3" one ($4.50) to my store, for pickup:
http://www.homedepot.com/p/Speedi-P...Degree-Adjustable-Elbow-EX-26A90-03/202907324