Try this once to see if you can narrow it down which rule set may be causing issues. Go into the Intrustion Prevention section of Blink. Start by going under the "Signatures" tab section. Uncheck each section of rules one by one and try downloading your mail after each one. See if you can find a particular section of rules that may be affecting your mail. If after you find a section that is causing this, open up the section and then disable each individual rule under that section until you pinpoint the exact one that is causing the issues.
You may also try this under the Analyzers tab section too, however, I doubt that is the issue.