Canon BJC-3000 - Manual (Page 249)

249 Troubleshooting - Paper Jams or Does Not Feed
Envelopes do not feed
Check 2 Are too many envelopes loaded?
The number of envelopes you can set in the auto sheet feeder is up to 10.
Do not exceed 10 envelopes in a stack.
Using Envelopes
Check 3 Are the envelopes loaded correctly?
To avoid misfeeds and jams, always load the top edge of an envelope (or
stack of envelopes) length ways with the flap side down.
Using Envelopes