Smearing while printing envelopes

[SYC_Auction]

I have a mail-merge document for printing envelopes for an event.  I've tried to feed the envelopes thru the external document feeder.  After two envelopes, it begins to smear badly.  

When I print a single envelope at a time, the print "motor" seems to run or turn for several seconds after the one envelope is printed; this is the same "motor sound" when printing any normal print job, but on this envelope, it continues to "run" after the printing is complete.

But when I print several envelopes at a time, the "motor" runs for the first one, then it's silent, though it's pulling the next envelope thru, and the next... but they're smearing.

Seems there is some resetting or cleaning going on when the "motor" runs.

I've tried to change the "paper" to an envelope, but it won't let me change it to the A-7 size as an envelope.

Help!  If I can't fix this, I'll have to pay a service to print these for our non-profit.  Tried our LaserJet Pro 200 color, but it couldn't pull the envelopes. 

[SHThompson · Accepted solution]

You mention size but are you setting the paper type to envelope?

The type is very important because envelopes are thicker and stiffer than regular paper and are normally fed slower and possibly even with a higher fuser temperature.