Inconsistent [From: External] appearing on emails

Our company sends out weekly emails using constant contact; despite that we set is so that the "From address" is the same each week and is an internal email address, about half the time Outlook marks these emails as "[From: External]" and half the time Outlook recognizes the emails as internal.

Because the emails themselves are consistent, and the email addresses are consistent, and the company Outlook rules are consistent- I wondering if the issue might be a setting within Constant Contact. Any advice or suggestions?