Some of our sales reps have noticed that [MARKETING] is being added to some of the subject lines that they're sending out of sales insight. Are there any ways or best practices to prevent this, or is it just a filter on the receivers end that's impossible to change?
Yes, this is a filter that is set on the receiver's end, but you can influence it, as it depends on various factors, including the subject and content of your email.
Try to make these sales emails look as much as possible as if a real human had written them in outlook. The writing style as well as the email template and the minimum use of images in the email will help.