Canon BJC-8500 - Manual (Page 67)

2 Raise and lock the envelope guides on both sides of the sheet feeder.
3 Make sure all the envelopes are pointing in the same direction and that
no flaps are overlapping. With the flap sides down and the tops of the
envelopes pointing down, insert the stack of envelopes into the sheet
feeder.
You can perform printing only on the address side of the envelope;
never attempt to print on the flap side of an envelope.
4 Align the stack of envelopes with the right edge of the sheet feeder,
and slide the stack in until it stops.
5 Do not load more than 15 envelopes at a time. Make sure the top of
the stack is not above the load limit mark on the sheet feeder.
62 Canon BJC-8500 User’s Manual
Do not exceed the line!