Because of United States export restrictions that limit the strength of cryptographic systems shipped outside the United States, international versions of Microsoft Outlook will support only CAST 40 encryption. Currently, DES and CAST 64 are available only in versions of Microsoft Exchange Server sold in the United States and Canada. In addition, some countries/regions, such as France, have their own restrictions on the use of cryptography. For example, Microsoft Outlook cannot use advanced security in France.
International organizations that use Microsoft Exchange Server in several countries/regions can mix and match types of encryption. This is possible because Microsoft Exchange Server maintains information in the directory about the type of encryption that is supported for every user in the organization. If a message is addressed to multiple recipients using different types of encryption, Microsoft Outlook automatically attempts to encrypt the message using the type of encryption that all recipients share. For example, if a message is addressed to a recipient using CAST 40 and another recipient using CAST 64, the message is encrypted using CAST 40. If a message is addressed to a recipient who is not using advanced security, Microsoft Exchange Server allows the user to either not send the message to that recipient or to send the message in plain text format.