资料题：It would appear from the text that avalanches are brought about because（  ）。

Unencrypted(未加密的) messages can be hijacked in transit and read or altered.If the mail is not digitally signed, you cannot be sure where it came form.There are many options for securing e- mail, all with a few strengths and probably more weaknesses.Let's take care of the easy decisions.Secure/Multipurpose Internet Mail Extensions (S/MIME) should be the message encryption and digital signature format because it' 8 the accepted standard and is built into leading e-mail clients such a Microsoft Outlook 98/2000 and Lotus Notes RS.Yet a standard such as S/MIME only takes you 60 far.Each vendor has implemented is own interpretation of S/MIME, which makes interoperability problematic.This drawback is exacerbated by the emergence of S/MNE Version 3 in the newest e- mail clients, which again could create interoperability issues.The path of least resistance is to get an e- mail security gateway, which is analogous to a firewall for e-mail.Every message going in or out passes through the gateway, allowing security policies to be enforced (where and when messages can be Bent), virus checking to be performed,and messages to be signed and encrypted.One drawback of the gateway approach is that it doesn't provide user-based security.For example, the gateway encrypts outbound messages so recipients can verify they came from your company, but recipients cannot prove from whom they came.Client- based methods use your private key to sign messages (proving it came from you),which is a more granular(细小的) level of security, but they have weaknesses as well.They need to be configured on each desktop, which includes issuing a digital certificate to each user(for encryption and digital signature), and ensuring that a proper security profile is configured within the e-mail client.There are also a number of Web-based secure mail services that keep all messages within their environment at all times to ensure security.You use a secure site on the Internet to compose a message.Once you hit "Send" , the site encrypts and stores the message on its site, and sends the recipient an e-mail notification that a secure message is waiting.The recipient links to the site, provides 8 shared secret for authentication(认证), and accesses the message via Secure Sockets Layer.Unfortunately, this method does not work with existing enterprise e-mail systems.