Some maintenance was being done on the mail servers, though it doesn't sound like you ran into that.
Of course, don't forget to check www.dreamhoststatus.com for the latest outages...
Other causes of missing e-mail are SPAM filters or server blacklists. Can you confirm either? Depending on the content of your test e-mails, a spam filter may have caught your message.
I've had clients complain about e-mails falling into the void on other services.. It wasn't happening often enough to troubleshoot and was a source of frustration for me and my client.