A method includes: generating object information that indicates an object designated from among a header item, text, and attached information of a received email, or feature amount information based on the object information and a predetermined function, when a source is an address in an internal network, decrypting verification information added to the received email using secret key information shared in the internal network, when the source is an address over an external network, decrypting the verification information using public key information shared with the source, and verifying whether or not the received email is a spoofed mail based on the object information or the feature amount information, and the decrypted verification information.