| Individual messages can be protected by requiring message signing at the creation point (Outlook), at the originator’s discretion, enabling integrity protection for their messages. However, messages can also be created by report generators and other applications using automated processes that do not typically sign messages.
By signing outbound messages as they exit into the public Internet, the sending SMTP server gives all receivers the opportunity to authenticate the sending domain and server as authentic. (using the DNS-based DKIM record), and validate the message content as unaltered in transit (using the DKIM public key to rehash). In this way, forgeries are prevented, SPAMMERs are more easily tracked. To be effective, it should be noted that unless both senders and receivers participate, sender authentication techniques are of limited effectiveness.
For receivers not configured to recognize signed messages, there is no impact to processing – they default to treating the messages as if from anonymous sender origin, and examine it with the evaluation methods that are available.
The DKIM (Domain Keys Identified Mail) process is not part of Exchange 2003 functionality; so inbound messages that reach an Exchange server as the first receiving touchpoint will not be able to perform this type of sender authentication. However, most e-mail Secure Gateway products now offer this feature.
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