Mail conversion processing device for safely transmitting mail attached with large file in internal network over security network with isolated internal network to external network, and its operation method

An electronic mail processing device located in an isolated internal network of a security network separated from an external network, the device comprising:

• an internal network link identification unit for identifying, when a transmission mail requested to be transmitted is acquired from a sender device located in the isolated internal network, link information of a large file located in the isolated internal network from the transmission mail;
• a target file download processing unit for acquiring the large file located in the isolated internal network on the basis of the link information;
• a general attached mail conversion processing unit for generating a conversion mail in which the large file is inserted to be classified as a general attached file of the transmission mail;
• and
• a conversion mail transmission unit for transmitting the conversion mail to a mail restoration processing device located in the external network via a network link mail approval device.