Apparantly the unbreakable encryption can be broken if Hushmail willingly hand over the key to the authorities:

 … it seems the Canadian company has been divulging keys to the American authorities.

The document describes the tracking of an anabolic steroid manufacturer who was being investigated by the Drug Enforcement Administration (DEA). The document alleges that the majority of those engaged in the trade in anabolic steroids use Hushmail to communicate.

The DEA agents received three CDs of decrypted emails which contained decrypted emails for the targets of the investigation that had been decrypted as part of a mutual legal assistance treaty between the United States and Canada.