It's always an issue that pops up, I've had many faults logged for wrinkled envelopes, in some cases its the construction of the envelope, in some it's the settings and in some it's the way the printer is used, all can work against getting a good print out.
First thing is to print from Tray 1 and where possible open the rear door, any route that curves the media path will increase wrinkles, and you have documented you're using Tray 1, next is to make sure the envelopes are the right construction, this has perhaps been the largest factor in good or bad printouts, many customers show the box stating the envelopes are laser endorsed or laser certified but if you count the layers you often see an odd number of layers on one side to another, I've sometimes proven this by cutting a thin sheet of paper laying it on the envelope to make the thickness even then processing it and it normally comes out wrinkle free.
There are one or two printers that handle envelopes better than others but in general since the day the Laserjet 1 hit the market this has been all I do to fault find envelope issues, media changes within the printer often help with toner bonding etc but I've often found not much change in the wrinkles, the largest changes are in the media path and the envelopes themselves, contrary to what many say expensive is not always best, I found some very expensive hand made laser envelopes to be some of the worst ever for wrinkling, while some cheap third party envelopes worked fine, some branded ones needed a "helper" layer of media to even them out as they had uneven layer construction.
The user manual should give you the media spec of the envelope for weight, thickness and layers, the normal rule is the same number of layers each side so if you count 3 on one side then the other can't be 1 or 2 as it will skew off when under pressure, often if you put a paper layer to even out the thickness you see a marked improvement and if so then it would lead to investigation of the envelope construction and supplier, printer changes would then help once you have found a good envelope to use.