Here's what worked for me, printing to a Lexmark E260dn from a MacBook Pro running Mac OS X 10.11.5 El Capitan. I don't claim it's the best; it's just the first thing I found that worked.
First, Lexmark is clear that they did not update the E260dn driver for El Capitan. Grrrr. Second, nothing I got from Gutenprint worked for this model. Waaaah.
These directions are a bit brief / terse, but hopefully people can follow them / fill in the detail, or find someone who can help them with that. I used the IP address of my printer (192.168.0.201). Obviously people should use their own.
1. In System Preferences / Printers & Scanners, delete the printer (select it in the left pane and click '-').
2. In a web browser, go to the printer management page (e.g. http://192.168.0.201).
3. Click the Settings tab, give your admin id and password, click Print Settings, click Setup Menu, change Printer Language from PCL emulation to PS emulation, click Submit. Click the Settings tab, click Print Settings, click PostScript menu, select Print PS Error, click Submit. Click the Settings tab, click Network/Ports, click Standard Network, unselect PCL SmartSwitch, unselect PS SmartSwitch, unselect Mac Binary PS, click Submit.
4. In System Preferences / Printers & Scanners, click '+' to add a printer, click IP, then in the address field enter the IP address of your printer (e.g. 192.168.0.201), from the Protocol dropdown select Line Printer Daemon - LPD, from the Use dropdown select Generic PostScript Printer, click Add, select Duplex Printing Unit, select OK.
You should be good to go. (I was 🙂.)