Generally good emails will not get blocked but on rare occasions this may happen and so to overcome this you can simply white list their email or domain and make sure an email sender never gets blocked by the spam filter.
First login to your spam dashboard: https://mailguardian.co.uk
If you need to retrieve login details for your spam quarantine see these instructions
Once you are logged into your spam quarantine dashboard go to “My Account > Whitelist” in the left menu
Click the “New” button to add an email to your whitelist and enter the email address you want to add to the whitelist and click save and close
