We had the same problem, and tried everything, including the above suggestion and also some suggestions found at [url]http://www.burnbright.co.nz/how-to-set-up-email-bounce-handling-using-silverstripe-and-cpanel/[/url].
For us, the problem turned out to be the DNS setup that we had done with our hosting provider. For example, if your website is www.xyz.com, and you have an outside provider to host the website, but handle email (@xyz.com) in-house or with a different provider, then you have to be careful how you set up subdomains or parked domains. In our case, the email function was actually working, but the web host's system was trying to take a shortcut with the delivery, saying "this email is addressed to someone at @xyz.com, and *I* am @xyz.com, so I won't bother going to the internet with it", and then of course the email is never delivered because this provider handles web only and not mail. The web host company had to make some more-granular settings related to our DNS handling and then all these kinds of email issues, including the SilverStripe admin emails, were fixed.