If feed envelopes through the MP tray make sure that the paper size and type setting on the machine is the correct envelope size and type.
On your printer driver, make sure you specify tray 1 or MP tray as the paper source.
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I have been printing envelopes for several years on this printer and never changed the settings, so I know they are correct. For some reason the printer acted up this time. After checking all the settings anyway, I left it alone to do other activity. When i came back in two hours, it had printed the envelopes-go figure. Thanks for your help.
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