Why can’t users register with a Rediffmail email address for their Zerodha accounts?
Zerodha does not allow users to register with Rediffmail email IDs due to issues with email delivery. E-mails sent from Zerodha are often not received by users who have registered using their Rediffmail email ID. It is important for users to go through all the mails from Zerodha, since all important communication is sent via e-mail.
Zerodha also uses emails to notify users of security events. Whenever a user logs in to their account from a new location, Zerodha sends an e-mail to ensure that the user is notified of possible breaches in their account. See Why did I get an email alerting me about a login from a different city? to know more.
If an account is already opened with a Rediffmail email ID, it is advisable to change it. To change the registered email ID, see How do I change my registered mobile number or email id with Zerodha?