Hello,
Thank you for reaching out to SmarterTools. What is likely happening is this user is failing DKIM or SPF. When this happens trusted sender status is revoked and the spam checks still apply.
There is a way around this however if you go to Settings->AntiSpam->Spam checks you can set up a custom spam check like the following.
Please let me know if this helps.
Kind Regards,
Zach Sylvester
Software Developer
SmarterTools Inc.
www.smartertools.com