Hi Krishna, my name is Neil, and I'm an Outlook user like you.
I'll be happy to help you with your question.
If you have unsubscribed to an email newsletter, the only way to re-subscribe to it would be to visit the website and sign up for the newsletter again.
There's no way to do this through Outlook, since newsletters and email subscriptions are handled by third parties who send out these to their subscribers.
So you would need to visit the notion.io website and locate the email subscription there. You might need to have an account or sign in and then select to receive this email.
If you find you are already subscribed to the newsletter. Unsubscribe from it and then re-subscribe to it.
I hope you find this information useful.
Kindest regards,
Neil