Ghost Text After Mailing

I'm a bit stumped as to what the real cuprit is hear but its a printing problem so hardware seems the best option.

Recently at work we had a couple letters returned as undeliverable and found that the toner seemed to have somehow transfered from one flap of the letter to another. We then printed some test letters using varying heat/toner output levels and mailed them to ourselves, all suffered the same issue to varying degrees with little to no correlation between settings and level of transfer. We did find that between some coarser and smoother paper we had the coarse paper for the most part seemed more effected, with much darker transfer than the smooth.

These letters look perfect coming out of the machine and all reasonable attempts to smudge the toner or generally reproduce the effect mirrored ghost effect within the office have failed, nonetheless once they have gone to the post office and come back they invariably have some level of transfer.

We are using an HP Laserjet 4250.

Any thoughts/suggestions/help are very much appreciated.

 

could moisture have gotten in there? due to the heat of the fuser, and the surrounding area, maybe there's fluxes in the temperature?