Content filtering built in to Exchange is kinda like free antivirus software. Big promises, but it never works like it says. It either blocks ham (good emails, opposite of spam) or not much of the spam. I use a third party content filtering service called GFI. I'm sure there are others out there, but I have little to no experience with them. If you do use a third party filtering service, you're going to want to disable the filtering built into Exchange, and here's how.
To disable filtering, open up the Exchange Management Shell and type:
Set-ContentFilterConfig -Enabled $false
When you disable content filtering, the underlying Content Filter agent is still enabled. To disable the Content Filter agent, run the command:
Disable-TransportAgent "Content Filter Agent"
If filtering is disabled and you'd like to enable it, type into the Exchange Management Shell:
Set-ContentFilterConfig -Enabled $true
If you're not sure whether it's on or off, you can check by typing:
Get-ContentFilterConfig | Format-List Enabled