Sending a posting declaration or certificate via SIPSI is mandatory, whatever the sector, the nature or the duration of the posting. The SIPSI service has been the only means for sending the posting declaration or certificate since October 1, 2016 (January 1, 2017 for the transport sector). Declarations sent via email, mail or fax are not acceptable. The absence of a posting declaration makes you liable for high fines and exposes you to the risk of seeing your service suspended in case of control.