We have 3 new WC 7220 printers and none will print #10 envelopes from tray 5 with Mac Word 2013. We can insert the media in the tray and the printer recognizes it as #10 envelopes. But when we send a print job from Mac Word. We get the error message that Resources required. No media in the tray. We are able to prin just fine from Windows 10 and Word. We installed the latest Xerox driver from the Xerox web site (XeroxPrintDriver.3.64.0_1572) and no driver for Mac OS 7 will install on El Capitan. I tried creating a printer with the Airprint driver with this machine but it's way to limited. I get the same results when trying to print envelopes with Apple Pages.
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Does Page Setup accurately display the Envelope selection? It often does not and causes this exact issue, and does so on Windows and Mac.
As for airprint, it is intentionally limited, Apple only allows the most basic so they don't have to support any features beyond duplex.