Canon CLC5151 Reference Manual page 121

Place the envelopes on a clean, level surface, and press all the way around the envelopes
by hand, in the direction of the arrows, to remove any curls. Repeat this step five times for
each set of five envelopes.
2
If you are using ISO-C5, COM10, or Monarch envelopes, hold down the four corners of the
envelopes firmly, so that they and the sealed or glued portion stay flat.
Flap
If you are using ISO-B5, Nagagata 3, or Kakugata 2 envelopes, load the envelopes without
folding them.
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Making Prints Using the Stack Bypass