An earlier unverified email, sent just hours after Wednesday's blast that left 12 dead, had claimed the attack for the Harkatul Jihad-i-Islami (HuJI).
Neither claim has been confirmed by police as genuine.
Internal Security Secretary UK Bansal told reporters that the latest email was being studied by intelligence agencies.
Written in Hindi, it said Indian Mujahideen had timed the blast to explode when the crowd outside the court would be at its largest. It offered no specific motive for the attack.
"We own responsibility for this," it said, while threatening to carry out another bombing at a shopping mall in Delhi next week.
Indian Mujahideen was blamed for the last major attack in the capital, a series of blasts in several upmarket shopping areas in 2008 that killed 22 people and injured nearly 100.
Wednesday's powerful blast ripped through a crowd of litigants queuing to enter the court complex in the heart of the Indian capital. Nearly 80 people were injured, many of the seriously.
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