Hi @MUHAMAD FADHLI BIN MOHD KENALI ,
Seems like "</strong>" is missing in your current code. Try using the html code below instead and see if it can work as you expect:
<p><div style="background-color: #ecc9c9; width: 100%; padding: 2pt; font-size: 10pt; line-height: 12pt; font-family: 'Arial'; color: black; text-align: left; border: 1pt solid #800000;"><strong><span style="color: #a52a2a;"> CAUTION: </span> External Sender. Please do not click on links or open attachments from senders you do not trust.</strong></div><br> </p>
I tested in my lab tenant, and it can work without affecting the font of email body:
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