Why are emails from some senders being classified as spam, despite adding them to allowed list?

Why are emails from some senders being classified as spam, despite adding them to allowed list?

If there is an SPF failure, and there are no actions set up for an SPF failure, emails from addresses present in the Allowed List might be marked as spam. The SPF failure might occur in case the email is sent from a spoofed email address. In such a situation, the email address/domain added in the Allowed List might not be effective. Learn more.