Canon BJC 3000 Reference Manual page 248

Envelopes do not feed
Check 1
Are the envelopes curled or bent?
Before you load envelopes, make sure they are flat. Press down firmly
around the edges of the stack to flatten the envelopes and expel air from
inside the envelopes.
If the envelopes are still not perfectly flat, hold the edges of the stack diago-
nally and bend them gently to straighten them.
Make sure that the flaps in the stack are straight and not overlapping.
If the envelopes still do not feed, use a pen or similar object to flatten the
edges of the envelopes.
Using Envelopes
Troubleshooting - Paper Jams or Does Not Feed 248