We've just implemented it at Sweego, but the mailbox providers haven't followed suit yet.
When an email becomes obsolete, what’s the point of keeping it in your inbox? With this expiry date, email software providers and editors will be able to enable their users to automatically delete expired emails, thus limiting their digital footprint.
Does your Email Service Provider offer this option?
To know more about Email expiration date, check the first comment ;)
Sweego