Hello Rijule.
Welcome to Apple Support Communities. We understand that you’re getting a notification Mail on your iPhone 6s and iPad Pro. We are happy to help.
Let’s use the steps here, make sure that you’ve checked that the date and time on the device are correct. If they are correct, you'll want to reach out to your email service provider to resolve the issue.
In the meantime, to stop prompts from appearing on the device, the customer can temporarily turn off affected services on the account (Go to Settings > Mail > Accounts > [Outlook] and toggle "Mail" off).
Take care.