Yes, it could be MI6, or the CIA.

Or the Keystone Kops.

But it's almost certainly Russia.

"Vitali Kremez, director of research with New York-based cyber intelligence firm Flashpoint, told Reuters his review indicated that APT 28, a group tied to the GRU, the Russian military intelligence directorate, was behind the leak. 

He cited similarities with US election hacks that have been previously attributed to that group."

http://www.telegraph.co.uk/news/2017/05/06/russian-hackers-blame-emmanuel-macrons-leaked-emails-could/

.