Email security in a multi-tenant email service

The technology described herein helps improve email security within a multi-tenant email service. In particular, the technology described herein helps make it more difficult for a first tenant of the email service to impersonate a second tenant of the email service by validating the sending domain before the email leaves the multi-tenant email service. In particular, the technology runs a sending-side source validation on an email. If the source validation fails, the email is still sent, but using a source IP address that will cause the email to fail a receiving-side email authentication tests, such as the sender policy framework (SPF) email authentication test. In contrast, if the source validation passes, the email is sent using a source IP address that will cause the email to pass the receiving-side email authentication tests.