Are there maximum Notification Volume Limits for Emails/SMS?

There are theoretically no limits from our end (subject to our own processing speed) in regard to the delivery of push messages and emails. However, there are limitations from backend providers, e.g. Google Firebase (see for instance https://firebase.google.com/docs/cloud-messaging/throttling-and-quotas#device_throttling ) and relaying/receiving SMTP servers that are not in our control. For SMS, there is a built-in flood protection per user, limiting the number of SMS message per minute to 5 (https://support.signl4.com/hc/en-us/articles/4406223295633-How-does-flood-protection-work). There is also an API limit from our messaging provider, ranging between 1 SMS per second to 100 RCS messages per second.