Iran's telecommunications ministry has barred local banks, insurance firms and telephone operators from using foreign-sourced emails to communicate with clients, a specialist weekly said on Saturday.
"The telecommunications minister has ordered the use of domain names ending with .ir" belonging to Iran, Asr Ertebatat reported.
The order prohibits banks, insurance firms and telephone firms using foreign hosts for their sites or to inform their clients using foreign providers such as Yahoo, Gmail, Hotmail or MSN, it said.
The weekly said that individuals seeking to communicate with such firms must now use email addresses ending with iran.ir, post.ir or chmail.ir.
Entities linked to the Iranian government must use addresses ending in gov.ir or .ir, while universities should use emails ending in ac.ir or .ir, the report added.
Iran has announced that as of May a national information network will be used to replace the Internet in the daily management of the administration of state entities, the banking system and public enterprises.
Officially, the launch of the "Iranian Internet" aims to secure communications by making them independent from foreign Internet operators.
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Iranian authorities announced in December having repatriated 90% of official websites and encouraged Iranian companies to do the same.
For the past two years, Tehran has been slapped with Western economic and financial sanctions due to its controversial nuclear programme.
The regime also regularly accuses the West of using the web for an "undeclared war" to destabilise it, and Telecommunications Minister Reza Taghipour has argued that Google and Yahoo posed a "threat to national security."
With over 36 million Internet users out of the population of 75 million, electronic media played a major role in the popular protests which rocked the country after the disputed re-election of President Mahmoud Ahmadinejad in 2009.
The authorities have since cut off or reduced Internet connections and speed.
The telecommunications ministry in April, however, denied that the authority has decided to cut outside Internet connections to support the development of Iran's own intranet.
Earlier this year, access to foreign-sourced emails was cut without explanation, disrupting the operations of many companies and millions of Iranians while prompting sharp criticism within the regime.
Since the unrest of 2009, authorities have sharply reduced the available bandwidth of the Internet and blocked access to tens of thousands of foreign websites, including opposition sites.
US President Barack Obama on March accused Iran of imposing an "electronic curtain" of censorship, announcing steps to use software and social media to help Iranians communicate online.