The last post (about the Arizona family and the teenager accused of criminal possession) reminds me that sometimes this situation might be viewed as a "strict liability offense", where the lack of evidence of intention or negligence might not be a sufficent legal defense. Sometimes certain acts or events incur "zero tolerance" strict liability because society considers the underlying risk so dangerous that it wants to compel people to play "brother's keeper."
Here, I wonder what the criminal liability could be when an unsolicited email is received, opened, and contains embedded HTML, images, or MIME that loads detectable illegal content onto the user's hard drive, detectable on warrant searches by police even if the user tries to delete the file (short of wiping out and rebuilding the hard drive). It is certainly a good idea to eyeball the subject lines of incoming emails and move suspicious emails to the spam folder (from the ISP) without opening them at all. It's a good idea to spam-classify emails with "no subject" (and I wish AOL and other ISP's wouldn't even permit accidental sending of emails with no subject).
One can even speculate about another theory, that a controversial user has "attracted" or "enticed" the sending of illegal content. Other than the Arizona case (previous post) I haven't heard of real prosecutions or civil suits based on this theory yet, but the possibility is chilling.
Credit card customers should also check their statements for charges that they do not recognize, and investigate and challenge unauthorized charges. People have been prosecuted for illegal possession based on credit card records, and presumably an imposter could make an illegal purchase on a stolen credit card number. Marketing companies have somehow been able to push "membership charges" onto many credit card bills by a process that escapes me, at least.
Be sure to visit the sidebar by Adam Liptak, "Locking Up the Crucial Evidence and Crippling the Defense: A law meant to protect children rewrites the rules for the accused." The law is the Adam Walsh Child Protection and Safety Act. The Govtrack page (109th Congress, HR 4472) is here.