Why do paper jams occur when feeding envelopes?
There are some types of envelopes which will cause paper feed problems due to the paper type, the age of the envelope and the environment they are kept in. (i.e too humid, too hot etc)
Try reducing the number of envelopes you set in the paper tray or make the surface of the envelopes as flat as possible.
If the problem remains, please try bending the leading edge of envelopes as shown in the image below.