Canon imagePRESS C800 Series Operation Manual page 151

After you finish
IMPORTANT
Make sure you put the envelope guide into its holder to avoid a paper jam.
Tightly rewrap the remaining media in the original package, and store the package in a dry place,
away from direct sunlight or high temperatures.
imagePRESS C800 Series
Load envelopes into the internal paper trays
Chapter 5 - Print the jobs 151