Smudgy printing, back of envelopes

[ChrisGreaves]

The printer is a HP Color laserjet 2600N.
I print envelopes with black ink.
The face-side of the envelope comes out looking good.
The backside comes out smudged.

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I suspect the smudging might be ralated to the flap of the envelope not providing traction during the feed process.
What's really annoying is that sometimes there's no smudging at all.

I'd be interested to hear of experiments I might perform to alleviate this problem.
P.S. I should add that printing black-ink on regular letter-sized paper, fed in single-sheet by the same front-orifice does not smudge.
That is, the problem seems to be restricted to the backside of envelopes.

[ChrisGreaves]

The backside comes out smudged.

Hmm. Maybe the answer lies in Printing Envelopes using Word 2010

"I use my printer usually set to 'fast draft' but when printing I need to operate it at 'normal print' to avoid smudging the envelope printing/printing too fast."