Canon LBP7210Cdn Manual page 248

"Paper Source"
NOTE
How to load envelopes in Drawer 2 (optional paper feeder)
Load the paper in Drawer 2 in the same manner as you load the paper in Drawer 1.
Step 1: Loading Envelopes in the Paper Drawer
Pull out the paper drawer.
CAUTION
Be sure to remove the paper drawer.
If paper is loaded while the paper drawer is partially pulled out, the paper drawer may drop or the
printer may become damaged resulting in personal injury.
Align the envelopes.
1. Place the stack of envelopes on a flat surface, flatten them to release any
remaining air, and be sure that the edges are pressed tightly.