Dr. David Jefferson is an internationally recognized expert on voting systems and election technology. He has been a pioneer in research at the intersection of computing, the Internet, and elections for 20 years, and has been an advisor to five successive Secretaries of State of California on technology-related issues. His full bio is available from here.
From a security point of view email voting is about the worst form of voting ever proposed. It is easy for many parties to read or modify ballots while in transit from the voter to election officials. It is also easy to simply block selected ballots from being delivered. Such attacks can be automated to affect a large number of votes, and can be perpetrated remotely, by anyone on Earth, including criminal syndicates, domestic partisans, or foreign intelligence agencies. Neither the voter nor the election officials can detect such attacks, let alone prevent or correct for them. Sending ballots by email is as dumb as taping a $100 bill to a postcard and expecting it to be delivered safely. Basically, it is naïve and irresponsible to send any kind of secure or confidential document by ordinary email.
In more detail, here are the technical facts.