Brother MFC-8820DN - Manual (Page 171)

11 - 14 USING THE MFC AS A PRINTER
5
Put the envelopes in the MP tray so that they touch the back of
the tray. Do not put more than 3 envelopes in the MP tray at any
one time, or it may cause a jam.
Make sure that the envelopes are neatly stacked and in the
proper position in the MP tray. If they are not, the envelopes may
not be fed properly, resulting in a skewed printout or a jam.