I would not send that to guests...
Here is what I do (took a few ideas from other hosts, and came up with this)... After the guest leaves (same day or the next) I send them a message (sorry for the language mistakes, my original one is in portuguese).
Hi X,
Did you have any problems with the check-out? Thank you for respecting our space during your stay and for leaving it clean and tidy. We really appreciate that! I hope you have enjoyed our city.
If you don't mind I would like to ask you for a favour if you have a few minutes. Opinions and grades have a great impact for both guests and hosts, if you could send us a review it would be greately appreciated.
Airbnb system is up to 5 stars...
5 Stars - Better than I expected (perfection doesn't exist but we were happy here)
4 Stars - As expected (There are a few things that need improving)
3 Stars - Below what I expected (There were some serious problems)
2 Stars - Way below what I expected (Close this host)
1 Star - Disaster! (Call the police and arest this host)
We work hard to get five stars, and we would really appreciate if you cold send us a message on how we are doing in that aspect, we are always looking for a way to improve guest experience.
In case you came back to our city we hope to see you again.
Thanks