From the Windows blog:
Cortana* already gives you reminders for people, places and times, and helps you make and manage to-do lists so you don’t forget a thing. Now, Cortana can help you remember things you’ve said you would do in your emails—without you even having to ask.
We’ve all been in the situation where we’ve promised to do something in email, like send your boss a report by end of day, or purchase tickets to the movies for your date night. But the day goes on, emails pile up, and you quickly forget. Now Cortana has your back. Using machine learning technology developed in partnership with Microsoft Research, Cortana automatically recognizes when you make a commitment in email messages and will proactively suggest a reminder to you to follow through at just the right time.
Cortana does all the work in the background making this helpful feature incredibly easy to use. To get started, simply send emails as you would normally and when you’ll do something, Cortana will recognize that and save the details in a suggested reminder for you. If you’ve specified a deadline in the email, Cortana will ping you before it’s due and save it in the action center. Other suggested reminders will be in the Cortana home. No need to copy her on the email or change how you do things today, she will adapt to you.
Cortana’s suggested reminders are available in the U.S. on Windows 10 with support coming to iOS and Android in the coming weeks. The feature will begin rolling out today and currently supports Outlook.com and Office 365 work and school email addresses with support for other email services coming soon.
To check the feature out for yourself, make sure you’re signed in to Cortana, and have given communications consent. Then check that either an Outlook.com or Office 365 work or school account is connected through the connected services section in your Cortana notebook. Try emailing yourself a commitment like “I will send you the report by Friday” and look out for that suggested reminder from Cortana.
And there's a short promo video to emphasise the new functionality:
All available in the USA at first, so doubtless the UK will get it over the summer, on Windows 10 on desktop and phone, on Android and iOS too (where installed).