Interesting
Would Normaly extend the ARC using the box on the original ARC certificate, however I think that if the aircraft and ARC was not avalble I would email the copy of the ARC with the extension box filled untill I could get my hands on the original.
In these days of email and electronic records the exact meaning of "original document" has become unclear, I know of one airline that is inside the EASA system that is authorized to carry all the documents electronically in the EFB.
I can see no reason that the email that you have been sent is on valid.