Which of the following is NOT a guideline for sending messages in MULES?

Using 10 codes is not a recommended guideline for sending messages in MULES. In a system focused on clarity and efficiency, it's important to prioritize communication that is easily understood by all recipients. 10 codes can vary in meaning depending on the region or department, leading to potential misunderstandings.

On the other hand, proofreading for spelling and punctuation ensures that messages are clear and professional, which is essential in law enforcement communication where clarity can impact the effectiveness of responses. Avoiding urgency asterisks helps to maintain a standard of communication that does not overly sensationalize a situation unless absolutely necessary. Additionally, steering clear of unnecessary polite phrases helps to keep messages direct and to the point, which is crucial in law enforcement communications where time and accuracy are of the essence.