SAN DIEGO — San Diego County jail inmates can receive letters in envelopes again, starting next week, under a federal judge’s ruling that a Sheriff’s Department policy limiting personal mail to postcards is unconstitutional.

The 3-year-old policy violates the First Amendment rights of a monthly journal, Prison Legal News, to send its periodicals, brochures and multi-page copies of case laws to county inmates, U.S. District Court Judge M. James Lorenz said in a decision issued Thursday.

The judge issued a preliminary injunction, ordering sheriff’s jail officials to suspend its postcard-only policy by May 21.

“Defendants shall not refuse to deliver correspondence sent to inmates at the County’s jails on the ground that correspondence is not written on a postcard,” Lorenz wrote.

The Sheriff’s Department enacted a policy in September, 2012 banning mail in envelopes unless it involved an inmate’s legal case. They can receive postcards, magazines, small-sized books and emails, but an email writer may send only two letters of one page each in a day.

The purpose of the policy was to reduce the amount of drugs or other contraband smuggled into the county’s seven jails, Lorenz noted in his decision.

A year after the ban on mail envelopes, sheriff’s officials said drug smuggling had dropped significantly. More than a dozen states enacted laws permitting postcard-only jail policies.

Prison Legal News filed a complaint on Oct. 9 to stop the San Diego County sheriff’s officials from enforcing the letter ban. The company’s books and periodicals on corrections news and law cases are sent to 2,000 correctional facilities across the nation, the judge said in his ruling. San Diego jail officials were returning the material back to the sender.

The Sheriff’s Department argued that the postcard policy enhanced jail security because guards spent less time opening envelopes and examining pages of mail.

Lorenz wrote that the Ninth Circuit Court of Appeal has upheld a First Amendment right for publishers to communicate with prison inmates, and four other district courts have struck down postcard-only policies.

The judge agreed with Prison Legal News that the postcard policy wasn’t a security issue, since jail deputies could still inspect mail for contraband.

He noted that the Federal Bureau of Prisons and the Los Angeles Sheriff’s Office, with the nation’s largest jail system, accommodate letter mail without compromising security.

Lorenz also supported the Prison Legal News complaint that the policy offered no way to appeal under a Fourteenth Amendment right to due process. The judge said sheriff’s officials have said they plan to re-write the policy to include due process. He ordered the county to confer with the publisher on an appeal process within 30 days.

The judge ruled against the publishing company’s claim of a First Amendment right to mail inmates one of its books that exceeds the allowed 6-inch by 9-inch size. Lorenz said jail deputies have a legitimate security interest in limiting the size of books because inmates sometimes soak books in toilet water and let them harden into a bludgeoning weapon.