Contract number: ECHA/2020/666  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. II.5.3. Submission of e-documents via e-PRIOR  1. If provided for in the special conditions, the exchange of electronic documents (e- documents) such as requests for services, specific contracts and 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 ). 2. The contracting authority takes the necessary measures to implement and maintain  electronic systems that enable the  supplier portal  to be used effectively. 3. 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.  4. 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. 6. 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. 5. 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. 1. The parties agree that any e-document, including related attachments exchanged  via  e-PRIOR :  (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. 2. 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 established between the parties’  back offices to allow electronic  25  II.5.4. Validity and date of e-documents 