Canon S400 Reference Manual page 36

Using Envelopes
3
Load the envelopes as follows.
Load top edges of the address sides (flap sides down) into the auto sheet
feeder and align the stack with the right side of the auto sheet feeder.
Slide the paper guide to the left side of the stack until it is sung.
Print side (address side)
The maximum number of envelopes you can load is 10 pieces.
Important
Printing on Paper and Other Media 36