Canon LBP7210Cdn Manual page 251

Slide the paper guide at the front center.
(1) Hold the lock release lever.
(2) Slide the paper guides to be aligned with the length of the envelopes.
IMPORTANT
Be sure to align the paper guides with the width of the envelopes.
If the paper guides are too loose or too tight, this may result in misfeeds or paper jams.
Hold down the envelopes, then set them under the hooks (A) on the paper guides.