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. II.5.3 Submission of e-documents via e-PRIOR  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 ). 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. The parties agree that any e-document, including related attachments exchanged via  e-PRIOR :  II.5.4 Validity and date of e-documents  (a) is considered as equivalent to a paper document; (b) is deemed to be the original of the document;  (c) is legally binding on the parties once an  e-PRIOR authorised person has performed the ‘sign’  action in  e-PRIOR  and has full legal effect; and  (d) constitutes evidence of the information contained in it and is admissible as evidence in judicial  proceedings. The parties expressly waive any rights to contest the validity of such a document solely on the grounds  that communications between the parties occurred through  e-PRIOR  or that the document has been  signed through  e-PRIOR . If a direct connection is establi shed between the parties’  back offices  to allow  electronic transfer of documents, the parties agree that an e-document, sent as mentioned in the  interface control document , qualifies as an  EDI message . If the e-document is dispatched through the  supplier portal , it is deemed to have been legally issued  or sent when the contractor (or leader in the case of a joint tender) is able to successfully submit the  18 