DESIGNATED AGENT- The limitations on liability established in this
subsection apply to a service provider only if the service provider
has designated an agent to receive notifications of claimed
infringement described in paragraph (3), by making available through
its service, including on its website in a location accessible to the
public, and by providing to the Copyright Office, substantially the
following information:
the name, address, phone number, and electronic mail address of the agent
...
To be effective under this subsection, a notification of claimed
infringement must be a written communication provided to the
designated agent of a service provider
At the same time, the actual form to designate an agent implies that the notification should be delivered by mail:
http://www.copyright.gov/onlinesp/agents/n/news_ycomb.pdf
That said, whether YC ignores a notice it gets by e-mail because it doesn't believe it was delivered properly, or because it simply chooses to ignore the notice, the result would seemingly be the same -- YC can be sued for copyright infringement.
If the motivation for complying (or appearing to comply) with the DMCA is to gain the liability protection, YC would want to be easy to contact and act on the notice no matter how they receive it.
If YC isn't actually worried about being sued, which seems reasonable for a site that hosts nothing but minimal discussion and external links, then urging potential notifiers to jump through hoops makes more sense. That minimizes the amount of notices they have to deal with, and their impact on the site.
I'd still be interested to know the legal basis for that policy. I asked about this once before, because having read the actual text of the DMCA it doesn't seem like such a policy would be sufficient to achieve the safe harbour protection, but I'm not a lawyer and I don't think I ever saw an answer on the earlier discussion. If anyone at YC would be willing to share any information they can about their rationale, it might be of general interest to anyone running sites with user-created content, which surely applies to many people reading HN.