print

Reboot and try again.

If there is a separate printing path for envelopes than from that of what you're testing the printing with—it's fairly common for there to be two printing paths here, one a more straight-through path for envelopes, and one more circuitous for printing standard paper pages. Test printing on an envelope from (for instance) iWork Pages and see whether that printing is also distorted. That'll tend to point to either a problem with the printer, or with the printing software.

Try removing and re-adding the printer.

Then contact the folks at Easy Envelope support, and/or the folks at the printer vendor, depending on which way the above printer testing has pointed...

Try going to System Preferences/Printers & Scanners, then control - click in the left panel, and reset the printing system. Add back the printer.