In his first public comment, Zierlein said the e-mail was accidentally forwarded to NFL general managers and Commissioner Roger Goodell after it had been sent to him May 18 by Steelers pro scouting coordinator Doug Whaley.
The e-mail reportedly contained obscene material.
"It was a gigantic error and 100 percent unintentional," Zierlein said after the Steelers finished their final voluntary practice of the week. "To this day, I don't know exactly what happened. I made an inexcusable mistake."
The e-mail message incensed Steelers chairman Dan Rooney when he was told about it shortly after he heard about the "naughty" message.
The Steelers have warned Zierlein about what they called "a violation of league and team policy." No details have been disclosed.