Ubuntu Linux 14.04 LTS, Turboprint 2.40-1, licensed.
Canon Pixma Pro 10S
Printing to A4 works fine.
If I print a DL envelope, it appears to work OK, the printer light flashes to show it is receiving data... but it prints nothing.
I have tried this both from OE Draw and capturing a screen shot then printing the resulting PNG in Image Viewer.
I have power cycled the printer & rebooted the PC.