Resources
Configuration guides that state the limits up front. Each one documents what the platform supports, based on the vendor's own documentation.
The four ways to send application email through Microsoft 365, what each one actually allows, and the limits Microsoft documents for them.
Read the guideThe difference between the SMTP relay service, Gmail SMTP and restricted Gmail, with Google's documented limits for each.
Read the guideThe full developer documentation, including every SMTP header and API endpoint, lives at omnivery.com/docs.