An encrypted message is only as secure as the algorithm that is used to encrypt it. The security of an encryption algorithm is measured by how easy it is to find the weaknesses in the algorithm that can allow someone to decrypt the message without its key. The most secure algorithm is one that can be cracked only by trying every possible key combination, a task that could take many lifetimes depending on the length of the key. The algorithms chosen for Microsoft Exchange Server, including DES and CAST, have been studied extensively by cryptography experts and have no known methods of attack other than a brute-force approach of trying every key.
Algorithms that use longer keys are generally more secure than algorithms that use shorter keys because there are more possible key combinations. For example, because 64-bit encryption is approximately 16 million times more secure than 40-bit encryption, it takes 16 million times longer to crack. In Microsoft Outlook the key for one message doesn't crack the key for another message because every message is encrypted with a unique key.