Track when users read an email

There isn't any method/functionality that can allow you to do this in O365. What the support folks probably meant was to utilize the same methods that marketing companies (and spammers) use - add an image or some other element to the email, which when opened will signal that the message has been looked at. You will either need to do this via add-ins or server-side by redirecting the mail flow, not recommended unless you absolutely must do this.