Acceptance Rate
Email acceptance rate measures messages a receiving server accepts. Define the measured stage: API acceptance and SMTP acceptance are different events.
Email acceptance rate measures the share of attempted recipient messages that a receiving mail server accepts. Define the stage being measured: a sending service accepting an API request is different from a recipient's mail server accepting the message.
What acceptance confirms
An SMTP acceptance confirms that a server has taken responsibility for the message at that stage. It does not reveal the final folder or whether a person read it. A later delivery failure is still possible. RFC 5321 describes the server's responsibilities after acceptance.
A platform's “delivered” metric may be derived from sent messages minus reported bounces. Check the event definition before treating it as a direct measurement of recipient-server acceptance.
Measure the same event over time
For a recipient-server acceptance rate, divide accepted recipient messages by attempted recipient messages and multiply by 100. Define how pending messages and retries are counted, and avoid counting a retried message repeatedly.
If acceptance falls, inspect response codes and affected recipient domains. A temporary rate limit, an invalid address, and a policy rejection need different responses. Review inbox placement separately after acceptance. A successful handoff cannot tell you where the message appeared.