I'm willing to bet that Google receive some of the best legal advice on the interpretation of the DMCA, and it seems to me that they do not normally accept notices via email.
From Google's old DMCA page[1] (cached 13 April 2013 - now directs to an automated system):
> To file a notice of infringement with us, you must provide a written communication (by fax or regular mail and not by email, except by prior agreement)
From Google's current support pages [2]:
> Send the written communication to the following address: <postal address> OR fax to: <fax number>
> To file a counter notification with us, you must provide a written communication (by fax or regular mail -- not by email, except by prior agreement)
That is probably in their interest, since they do get lots of valid takedown notices. For them it would be easier to streamline the process with email.
Mmmm, I wonder how "oops, it was caught in the spam filter" would work out? At least with mail delivery you can have confirmation of receipt. With email what's the guarantee that the person in charge would look at it?
The recipient has no obligation to act on these notices in the first place. Lost in the spam filter has the same result as purposeful inaction -- the service provider hasn't (by choice or otherwise) taken advantage of the offer of limited liability they could accept by disabling access to the listed content without a court order.
It really shows how badly the DMCA has been drafted then. It never actually defines what "electronic mail" means. This could actually be defined as a fax, because that transmits "mail", "electronically".
"So sue me" creates friction. If sending the request by mail or fax is too much trouble, arguing in court that emails must be accepted probably isn't attractive either.
You may have a hard time arguing they meant to require an email address, but at the same time, stuff sent there would not be valid.