Creating an Exchange / Outlook warning banner for external recipients

Many of the clients I work with have been taken an interest in implementing warning banners for all emails received from outside of their organizations in an effort to combat the ever increasing phishing emails that make it through their SPAM filters.  A great write up by Joe Palarchio can be found here:

Office 365 – Providing Your Users Visual Cues About Email Safety
https://blogs.perficient.com/2016/04/04/office-365-providing-your-users-visual-cues-about-email-safety/

The blog post provides a great description for the configuration but many of the clients I work with felt the colours and border weren’t as prominent as they would like so I’d like to share the HTML code I use to generate the following banner for anyone who may want to use it:

Note that this is a mail flow rule:

The following is the HTML code:

<div style='border:dotted #003333 1.0pt;padding:1.0pt 1.0pt 1.0pt 1.0pt'>

<p class=MsoNormal style='background:#F4DF11'><span lang=EN-GB
style='font-size:10.0pt;font-family:"Cambria",serif;color:red;mso-ansi-language:
EN-GB;mso-fareast-language:EN-GB'>CAUTION:</span><span lang=EN-GB
style='font-size:10.0pt;font-family:"Cambria",serif;color:black;mso-ansi-language:
EN-GB;mso-fareast-language:EN-GB'> This email originated from outside of the
organization. Do not click links or open attachments unless you recognize the
sender and know the content is safe.</span><span lang=EN-GB style='font-size:
12.0pt;font-family:"Times New Roman",serif;mso-ansi-language:EN-GB;mso-fareast-language:
EN-GB'><o:p></o:p></span></p>

</div>
<BR>

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Note that my experience with the implementation banner was well received by IT infrastructure teams but often poorly received by users because such a banner fills up the body preview on smart phones rendering that feature to become useless so it would be wise to pilot this with users before implementing it globally.