English to Chinese Tech/Engineering Law: Contract(s) 合同

Except as otherwise specified in this Contract, any notice sent by registered post with acknowledgement due or special courier shall be deemed received 10 (ten) Days after dispatch. In proving the fact of dispatch, it shall be sufficient to show that the envelope containing such notice was properly addressed, stamped and conveyed to the postal authorities or courier service for transmission by registered post with acknowledgement due or special courier. 除本合同另有规定外，通过挂号信以确认或特快专递方式送达的任何通知应被视为发出后10（十）天内收到。为证明发出的事实，应足以表明含有该通知的信封上的地址是正确的、加盖了章印并传达给邮政部门或快递服务与确认因或特殊快递挂号邮递方式传送。courier service for transmission by registered post with acknowledgement due or special courier.这句是什么意思?
acknowledgement due 是什么意思?