The 6510 doesn't have the fuser with this option.
Here is how to do it. Give it a couple secs to load
Unfortuantly all laser printers struggle with envelopes.
All the new versalink A4/desktop printers do not have a straight path for envelopes so you'll probably get some curl. They DO have a cam setting on the fuser to uncam it slightly for printing envelopes without wrinkles. The envelopes should be rated for laser printing though.
You might want to talk to a sales person in your area, bring some envelopes to a demo room near you and test them out.
Want to buy the VERY best laser printer for #10 envelopes: what is Xerox's most flawless envelope printer (feeder). Optimally, a straight-feed, if Xerox has one, where envelope passes straight through to the output tray. We have owned both a cheap Xerox 4500 and a more expensive Xerox 7700...the 4500 did much better with envelope printing! (Those printers were burned up with our office in the Santa Rosa fires in late 2017; we now need new printers.) Our output per month is in batches of 200 and we print about 700 to 1,000 envelopes per month. A dedicated color envelope machine is OK if that is the best machine; but we would then also need to buy a color printer for paper printing. Past envelope printing problems were (1) crinkled envelopes and (2) periodic (often) jamming. We would prefer an envelope feeder of greater than 50 capacity for greater feed reliability and convenience. Thank you for your time.