A remote maintenance system operable to report fault information to a maintenance center device by electronic mail through the internet when a fault is detected by a surveillance unit mounted in a user device, the system comprising the user device and the maintenance center device, wherein the user device is provided with a connect confirmation unit operable to prepare and transmit connect confirmation mails of a plurality of patterns different in mail content, and is configured to transmit the connect confirmation mails from the connect confirmation unit in a given order, characterized in that: the patterns comprise check patterns corresponding to given limitations of the mail transmission environment of the user device, which limitations determine whether or not a mail with a particular such pattern can be transmitted by the user device or received by the maintenance center device, the check patterns being such that a said connect confirmation mail having a said check pattern will not be received by the maintenance center device if the mail transmission environment of the user device has the corresponding limitation; the maintenance center device is provided with a diagnosis unit operable to diagnose whether there is a given limitation to the mail transmission environment of the user device from a diagnosis file, in which is registered a criterion detailing diagnosis results corresponding to different cases in which said connection confirmation mails are received or not received, and from the connect confirmation mails received from the user device and received within a time-out time period, the period being started upon receipt of a first said connect confirmation mail, and operable to transmit a said diagnosis result to the user device or an arbitrary address by an electronic mail; the plurality of patterns are different in either of the presence or absence of a mail divided into parts, a mail size, an attachment file size, the number of attachment files, the extension code of the attachment file name or the length of the attachment file name, the attachment files being obtained from storage during such preparation; and the diagnosis unit, when the connect confirmation mails of all the patterns from among the connect confirmation mails of the plurality of patterns transmitted from the user device are received, transmits a connect success message as a diagnosis result based on the criterion registered in the diagnosis file, and when there are connect confirmation mails not received, transmits a message requiring a particular change of the setup of the mail transmission environment as a diagnosis result based on the criterion registered in the diagnosis file, to make mails receivable. the patterns comprise check patterns corresponding to given limitations of the mail transmission environment of the user device, which limitations determine whether or not a mail with a particular such pattern can be transmitted by the user device or received by the maintenance center device, the check patterns being such that a said connect confirmation mail having a said check pattern will not be received by the maintenance center device if the mail transmission environment of the user device has the corresponding limitation; the maintenance center device is provided with a diagnosis unit operable to diagnose whether there is a given limitation to the mail transmission environment of the user device from a diagnosis file, in which is registered a criterion detailing diagnosis results corresponding to different cases in which said connection confirmation mails are received or not received, and from the connect confirmation mails received from the user device and received within a time-out time period, the period being started upon receipt of a first said connect confirmation mail, and operable to transmit a said diagnosis result to the user device or an arbitrary address by an electronic mail; the plurality of patterns are different in either of the presence or absence of a mail divided into parts, a mail size, an attachment file size, the number of attachment files, the extension code of the attachment file name or the length of the attachment file name, the attachment files being obtained from storage during such preparation; and the diagnosis unit, when the connect confirmation mails of all the patterns from among the connect confirmation mails of the plurality of patterns transmitted from the user device are received, transmits a connect success message as a diagnosis result based on the criterion registered in the diagnosis file, and when there are connect confirmation mails not received, transmits a message requiring a particular change of the setup of the mail transmission environment as a diagnosis result based on the criterion registered in the diagnosis file, to make mails receivable.