In a large-scale mail system such as that of a mobile phone carrier, in case that a mail gateway has encountered a fault due to a fault-inducing mail in arrival thereat, the mail gateway identifies the cause of the fault to prevent the recurrence thereof. The mail gateway is configured to include a mail relaying process used for relaying mails and a monitoring process used for supervising other processes. On occurrence of a fault due to a fault-inducing mail received by the mail relaying process, the monitoring process detects the fault-inducing mail that has cause the fault, the characteristics of the fault-inducing mail and the conditions of the fault, and then records information thus detected into a fault-inducing mail information table. After recovery from the fault under control of the monitoring process, the mail relaying process receives a new arriving mail. Upon receipt of the new arriving mail, the mail relaying process compares the characteristics thereof with relevant information recorded in the fault-inducing mail information table. If a match is found, the mail relaying process performs one of three countermeasures according to the conditions concerned; skipping over a faulty-point processing, returning an error reply to a mail transmission source, or returning a notification of non-delivery thereto.