To access your Rediffmail account from Gmail, you need to follow these steps:
Open your Gmail account.
Click on the "Settings" icon and select "Settings."
Click on the "Accounts and Import" tab.
Click on "Add a mail account" under the "Check mail from other accounts" section.
Enter your Rediffmail email address and click on "Next."
Enter your Rediffmail password and click on "Next."
Select the options for importing mail and click on "Add Account."