We're stuck! After trying 4 different brands of standard #10 business envelopes, 24 lb paper, nothing special... Not one envelope has ever printed successfully. Yes we set the #10 envelope in as the paper size, and we're using tray 2 (with that paper indicated). No matter what, the envelope feeds up through the part that you access by opening the top door and pulling out that carriage toward the back, then through to the rollers in front of that. What you see is the envelope jammed up and crooked. We have placed just one single envelope into the tray.
Well after owning this printer for 4 months, and fighting with envelopes multiple times and then giving up in disgust each time, Stevie happened to try using Tray 1. The guides are a pain to move with the little blue thing, but the envelopes print OK!!! So we're off and running. No need to change the paper type either (or get the rude little message if you forget) and #10 is not available for tray 1 anyway... Just load the envelope in Tray 1, print to Tray 1, and be happy - ish.
PS: Not accepting as solution because it is designed to print envelopes from tray 2, documented that way, but simply broken.