Detailed answer
You need a PAN and access to the e-filing portal. Aadhaar OTP works only if your Aadhaar is linked to a working Indian mobile number.
If the return is not verified within 30 days, the date of verification is treated as the filing date (a late fee can apply), and an unverified return is treated as not filed.
Indus Tax prepares and files NRI returns remotely and walks you through e-verification from wherever you are.
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