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To print envelopes of different sizes.. What you need to do first is remove the lower tray. remove the paper.
Insert your envelopes into the tray. Open / Close the plastic guides so they are holding the envelopes in place properly.
Here is a visual guide showing how to load the envelopes. http://ugp01.c-ij.com/ij/webmanual/Manual/W/MX920%20series/EN/BG/Bg-Loading1500.html
Once thats done, you need to change the paper / media settings in the print driver.
Then, you just need to open the printer driver setup window, select envelopes, and then change the size of the envelope on screen to match the size you just loaded. This page shows you how and where to do that.
If you have further questions, let me know.
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