MX Guarddog offers email protection to deal with modern email challenges. 
Spam and virus protection for businesses, organizations, and Internet domain name owners.
MX Guarddog will filter incoming email, forwarding only clean email to your mailboxes – for free!
MXGuarddog does require you to place a link on your web site in return for this service. Or for five dollars you can purchase “twenty credits” for the service. In my experience one credit is equal to one email address scanned for spam messages per month.
The MX Guarddog SPAM and anti-virus protection service is used by companies who manage their own email servers but either don’t have or do not wish to pay for a quality SPAM & antivirus solution. The MX Guarddog service is available to any company, organization or individual who owns their own domain and would like to filter email before it reaches their destination mail server. This will also work with any email service that you use.
You can retrieve messages marked as spam by MX Guarddog through your domain dashboard. Any messages that contain a virus will be marked by a lady bug in the subject column.
There are three filter options for this spam and virus solution. A blacklist, whitelist and attachment filter are available to configure to your specifications. The domain blacklist can filter email by email address, domain name, country of origin or a network range. The whitelist can be used for email addresses, domain names, network ranges or subjects. The attachment filter will set which attachments to remove no matter where the message originated.
Setup for a domain takes about 30 minutes. A video on the site steps you through the setup process for your domain. Try this external service and compare it to your current spam protection.