Thank you for you post. On rare occasions I have seen this code under different circumstances than what is listed.
Sometimes this code will result due to a message being sent with an attachment that may be executable. For example sending a .EXE file.
Sometimes, this code will generate because someone in your recipient list is no longer a valid contact and is causing the bounce to occur.
I have also seen this occur when replying to a message with a log string of content.
Try some of the following to help alleviate the issue and*or narrow down the cause.
1. Try sending emails from webmail instead of using an email program such as outlook or mac mail to see if this occurs.
2. Instead of replying to an email, try composing a new message and manually enter the recipients.
3. If you are sending attachment, try compressing the files first in such a format as .zip
If you have any concerns or questions feel free to reply with @TechFly