Can Siri read your emails to you? How to listen to emails on your iPhone - why Siri isn't you best bet
Ask Siri "read me my emails" to hear your first 25 email subjects and senders. To get the full text of the email you need to ask "read my last email", but you have to repeatedly push the listen button and ask for emails to be read out one at a time - using the same format, such as "read my second email". There's no way to have Siri read the next email or to interact in the context of the current email (eg archive or flag).
Speaking Email on the other hand is specifically designed for reading out emails. Unlike Siri, it does not distract you with voice control or require your attention, but intead provides a great email client using your phone's native text-to-speech capability and a focus on safety while driving.
We tailor the experience to the medium and the context. For example with email we detect legalese disclaimers, email signatures and other common email clutter and skip over them. Siri won't do this because it needs to handle a wide range of tasks.