Contract conditions of December 2015  Contract number: [ complete ]  E-mail is deemed to have been received by the receiving party on the day of dispatch of  that e-mail, provided that it is sent to the e-mail address indicated in Article I.8. The  sending party must be able to prove the date of dispatch. In the event that the sending  party receives a non-delivery report, it must make every effort to ensure that the other  party actually receives the communication by email or mail. In such a case, the sending  party is not held in breach of its obligation to send such communication within a specified  deadline.  Mail sent to the contracting authority is deemed to have been received by the contracting  authority on the date on which the department responsible referred to in Article I.8  registers it.  Formal notifications  are considered to have been received by the receiving party on the  date of receipt indicated in the proof received by the sending party that the message was  delivered to the specified recipient. If provided for in the special conditions, the exchange of electronic documents (e- documents) such as invoices between the parties is automated through the use of the  e- PRIOR platform. This platform provides two possibilities for such exchanges: either  through web services (machine-to-machine connection) or through a web application (the  supplier portal ). II.5.3. Submission of e-documents via e-PRIOR  The contracting authority takes the necessary measures to implement and maintain  electronic systems that enable the  supplier portal  to be used effectively. In the case of machine-to-machine connection, a direct connection is established between  the parties’  back offices . In this case, the parties take the measures necessary on their side  to implement and maintain electronic systems that enable the machine-to-machine  connection to be used effectively. The electronic systems are specified in the  interface  control document . The contractor (or leader in the case of a joint tender) must take the  necessary technical measures to set up a machine-to-machine connection and at its own  cost.  If communication via the  supplier portal or via the web services (machine-to-machine  connection) is hindered by factors beyond the control of one party, it must  notify  the other  immediately and the parties must take the necessary measures to restore this  communication. If it is impossible to restore the communication within two working days, one party must  notify  the other that alternative means of communication specified in Article II.5.1 will be  used until the  supplier portal  or the machine-to-machine connection is restored. When a change in the  interface control document  requires adaptations, the contractor (or  leader in the case of a joint tender) has up to six months from receipt of the  notification  to  implement this change. This period can be shortened by mutual agreement of the parties.  This period does not apply to urgent measures required by the security policy of the  contracting  authority  to  ensure  integrity,  confidentiality  and non-repudiation  of  information and the availability of  e-PRIOR , which must be applied immediately. II.5.4. Validity and date of e-documents  17 