Canon LBP7210Cdn Manual page 250

CAUTION
Be careful when handling envelopes.
Handle envelopes carefully not to cut your hands with the edges of the envelopes.
IMPORTANT
Loading orientation of envelopes
See the instruction described at
Load envelopes with the front side (non-glued side) facing up.
You cannot print on the reverse side (glued side) of envelopes.
Paper drawer capacity for envelopes
Approx. 10 sheets
Align the paper guides with the envelopes.
Slide the side paper guides.
(1) Hold the lock release lever.
(2) Slide the paper guides to be aligned with the width of the envelopes.
The side paper guides move together.
the top of the procedure.