Nothing really prevents this; a company I work with does something similar, and as long as you engineer your use of the platforms to be flexible and not rely on platform-specific features (for example MailChimp and SendGrid use slightly different templating approaches, and support different amounts of logic in the templates), it's doable.

This is the same idea as making your system work across availability zones/datacenters/cloud providers. At a certain scale it becomes a good idea.