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Message Body Partially Truncated Due to "NUL" Control Character

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In an Exchange 2010 environment mailbox users are receiving SMTP e-mail messages from a remote third party SMTP host. However when opening the emails in Outlook client and Outlook Web App (OWA) the message body is partially truncated.


  1. Captured affected e-mail messages at the inbound Exchange 2010 Hub Transport server using Pipeline Tracing.
  2. Analyzing the .eml file of the captured emails with a Hexeditor showed the body contains a "NUL" (0x00) control character, which causes the remaining part of the body to be truncated in Outlook.
  3. According to the RFC-2821 the "NUL" control character is allowed, but may cause problems and SHOULD be avoided: DATA (DATA)
    The receiver normally sends a 354 response to DATA, and then treats the lines (strings ending in <CRLF> sequences, as described in section 2.3.7) following the command as mail data from the sender. This command causes the mail data to be appended to the mail data buffer. The mail data may contain any of the 128 ASCII character codes, although experience has indicated that use of control characters other than SP, HT, CR, and LF may cause problems and SHOULD be avoided when possible.
  4. If unable to modify and avoid the sending SMTP host to insert the "NUL" (0x00) control character, as a workaround user can configure a Transport Rule on the Exchange 2010 Hub Transport server to add a disclaimer to the body of affected emails which are sent from this particular remote sender or sender domain. Once the Transport Rule touches the email, it removes the "NUL" control character and the full body is visible in Outlook and OWA.
When creating the disclaimer transport rule in Exchange Management Console, below rule settings could be used:
  1. Conditions:
    1. Enable when the From address contains specific words.
    2. Enter the sender SMTP address used by the remote SMTP host.
  2. Actions:
    1. Enable append disclaimer text and fallback to action if unable to apply.
      • For position use append.
      • In Disclaimer text box enter some text to verify the rule is working. If required later user can change the text and just enter one single space character if user actually does not want to add anything visible in the body.
      • For fallback use wrap.
  3. Exceptions:
    User can skip and just click Next.


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