Interesting use case. On Elastic Cloud, emails must be sent through EC servers and recipients must be whitelisted. Given those requirements, one possible workaround would be to setup your watchers as webhook actions instead: https://www.elastic.co/guide/en/x-pack/current/actions-webhook.html
This would require developing your own outside microservice for the watcher to hook into. The microservice would then be responsible for sending out the customer emails.
Of course, you could always ask your customers to kindly accept being whitelisted by Elastic Cloud 