Default message format must be set to use Plain Text.

From Microsoft Outlook 2013 STIG

SV-54062r1_rule Default message format must be set to use Plain Text.

Vulnerability discussion

Outlook uses HTML as the default email format. HTML format poses a security risk by embedding information into the email itself, which could allow for release of sensitive information. If a user attempted to insert an HTML link into an email message, the link itself may direct to a malicious website. By sending in that format, the recipient would be subject to becoming infected by the malicious website.

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Fix text

Set the policy value for User Configuration -> Administrative Templates -> Microsoft Outlook 2013 -> Outlook Options -> Mail Format -> Internet Formatting -> Message Format "Set message format" to "Enabled: Plain Text".

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