He said that striking the right balance between privacy and the government's ability to access information is "one of the most important conversations I think we can have in the 21st century."
Holder was responding to questioning from Rep. Suzan DelBene (D-Wash.), who has co-sponsored legislation along with Reps. Zoe Lofgren (D-Calif.) and Ted Poe (R-Texas) that would enhance privacy protections for online content.
Under the Electronic Communications Privacy Act (ECPA) of 1986, police only need a subpoena, issued without a judge's approval, to force service providers to turn over emails that have been opened or that are more than 180 days old.
Privacy groups and many lawmakers argue the law is woefully out of date and that police should need a warrant based on probable cause before accessing private online content, regardless of how old it is.
Holder's comments reiterate the Department's position that Elana Tyrangiel, the acting assistant attorney general for the Office of Legal Policy, laid out during a House hearing earlier this year.
She said at the time that there is "no principled basis" for the 180 day distinction and that legislation to expand ECPA's protections has "considerable merit." But she warned that because civil regulators do not have access to warrants, which are reserved for criminal cases, the legislation could impede their critical investigations.
The Senate Judiciary Committee voted to advance email privacy legislation last month without a carve-out for civil probes. The House bill from DelBene and others would also require a warrant for a phone's geolocation data.