From the User Guide:
Currently this machine supports the following European and Asian envelopes for use through the Bypass Tray:
Chau-kei3 (120 x 235 mm, SEF) 4.7" x 9.25"
Kaku-ga (240 x 332 mm, SEF) 9.5" x 13"
C4 (229 x 324 mm, SEF) 9" x 12.75"
C5 ( 162 x 229mm, SEF) 6.4" x 9"
The users I know running any envelopes on this machine have had mixed results. It all depends on what you consider acceptable quality. You will probably see some wrinkling and skewing, but it is possible to do. As with running any materials through your machine, use common sense and don't do anything that can have an ill affect on your service agreement.
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It all depends on what kind of envelope stock you use. I have found that some envelopes work and don't wrinkle but others wrinkle very badly. It's all trial and error