I used https://esputnik.com/ like in this post.
Also I used this script via google tag managet to send events to service
<script>
function sendEventForToken(pushToken) {
es('sendEvent', 'name_of_event', pushToken);
}
es('getPushToken', sendEventForToken);
</script>
You may want to do a domain spam check and file an appeal with Google support with some test emails. If your domain is a G Suite/Google Apps account, the domain administrator usually has access to 24/7/365 support from Google - see this page
The best Email Service Provider: