As you receive a reservation from Booking.com, you may notice that it has a different format of guest email with the following domain: @guest.booking.com
Booking.com uses alias for e-mails for guests avoiding any possible spam and keeping the guests privacy in place. Partners can use the alias e-mail to reach out to the guest normally. The alias email will be deactivated a few days after guests check out.
Whenever a property send an e-mail to an address as the example bellow, the guest will receive it in their inbox, normally.
Properties can use the alias email structure outside of the channel extranet to communicate with guests, this is fully supported.
According to Booking.com, there is no possibility for them to disclose this information due to compliance reasons.
You can check booking.com if the message was sent or check in MFD for email logging to see if the email was sent.