Brother MFC-3340CN Service Manual page 24

Do not use paper or envelopes:
n that are damaged, curled, wrinkled, or irregularly shaped
0.04 in.
or longer
n that are extremely shiny or highly textured
n that were previously printed by a printer
n that cannot be arranged uniformly when stacked
n that are made with a short grain
Do not use envelopes:
n that are of a baggy construction
n that are embossed (have raised writing on them)
n that have clasps on them
n that are not sharply creased
n that are preprinted on the inside
0.04 in.
or longer
2-5
Confidential