Assuming single sided output, that would be toner on a roller, and to get it there the toner would have to be dropping when the paper isn't there, which would imply you also have a printer that is going past the margins, or was.
You will need a tech, or take it apart and figure out how the roller is being contaminated.
Hi,
My work center printer is leaving ink smears on the back side of every paper. I have already tried reinstalling the toners. Any next steps to solving this?
~Jing