I assume you have the envelope fuser installed when running envelopes, correct?
We regularly run #10 envelopes on our C7090 out of tray 3. In fact, we have tray 2 turned on as well (DIP Switch 71-1, set to 1) so we can load one drawer while it's running out of the other. We run them short edge first, and you need to use side-seam envelopes
for best results. Make sure you open the small paper guides . You should also install the side auxiliary guides [1,5] (see image below). That's all we ever do, however, the manual also suggests installing the trailing edge auxiliary guides [4,6], and change the side air opening retainer levers . We never need to do those extra steps, but it might be worth trying since you're having issues.
If we get the occasional batch of envelopes that won't feed, we will rotate them 180°.
Lastly, because we have numerous finishers on our machine, we output the envelopes to the relay unit top tray. We take off the tray so they stack a bit higher. We found they are prone to getting jammed inside the SD-513 booklet maker if we have them exit all the way to the final finisher.
Interesting note about the side-seam envelopes...once we switched to them, an account called and asked that we always use those envelopes for their future orders. They were using them in a inserter/sealing machine and found the side seam ran much better for them as well.
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