A non-transitory computer readable recording medium including instructions, when executed by a processor, causing an email communication apparatus that is connected to an email server to execute processes comprising:
- receiving an email of a first type through an email server to which the email communication apparatus is connected from a transmission source device, the email of the first type designating a first email address as a transmission destination of a first acknowledgement email of the first type, the first type defining the email which is to be transmitted through the email server according to a Simple Mail Transfer Protocol (“SMTP”);
- storing the first email address in the memory in response to receiving the email of the first type;
- establishing a connection between the email communication apparatus and the transmission source device not through the email server;
- storing an internet protocol (“IP”) address of the transmission source device in the memory in response to establishing the connection;
- receiving an email of a second type not through the email server from the transmission source device using the connection, the email of the second type designating a second email address as a transmission destination of a second acknowledgement email of the first type, the second type defining the email which is to be received or to be transmitted not through the email server according to a direct SMTP;
- storing the second email address in the memory in response to receiving the email of the second type;
- determining that the email of the first type is not received if the IP address is stored in the memory and that the email of the second type is received if the IP address is stored in the memory;
- upon determining that the email of the first type is received, transmitting, through the email server according to the SMTP, the first acknowledgement email of the first type in response to the email of the first type, the first acknowledgement email of the first type designating the first email address stored in the memory as the transmission destination;
- and
- upon determining that the email of the second type is received:
- transmitting a specific packet to the transmission source device using the IP address stored in the memory;
- determining whether the email communication apparatus receives a reply packet in response to the specific packet;
- establishing a connection between the email communication apparatus and the transmission source device using the IP address stored in the memory upon determining that the email communication apparatus receives the reply packet;
- in response to establishing the connection using the IP address stored in the memory, transmitting, to the transmission source device not through the email server according to the direct SMTP using the established connection, a third acknowledgement email of the second type in response to the email of the second type, wherein authentication information with which the transmission source device performs authentication of the email communication apparatus is transmitted not through the email server before the third acknowledgement email of the second type is transmitted, and upon determining that the email communication apparatus receives, from the transmission source device, information which indicates that an authentication process using the authentication information is successful, the third acknowledgement email of the second type is transmitted not through the email server;
- and
- upon determining that the email communication apparatus does not receive the reply packet even upon determining that the email of the second type is received, transmitting, through the email server according to the SMTP, the second acknowledgement email of the first type in response to the email of the second type, the second acknowledgment of the first type designating the second email address stored in the memory as the destination.