I think the above answers are correct but this cleans things up and gives more info. As this is 2020, you the below search is not bound by Splunk 6.x or 7.x- it should work universally.
You must remember that there are also fields:
Which you cannot get by REST.
And of course those can be a email aliases/mailing lists on mail server(s).
One way which show to you where splunk has sent alerts is query those from internal logs.
index=_internal source=*python.log sendemail "Sending email"
Doesn't look like that does the trick. I ran with the email address we were looking for and and a couple of others but was unable to get an results returned. Also ran the search with an asterisk for the email.to and found that the search didn't appear to return an email at all. Thank you for the suggestion.