In order to provide a cost efficient filtering service to you, we are unable to open a dialog with customers regarding missed spam. Instead, we've created a process by which missed spam is to be reported which will help us improve your filtering and get you the explanation you're seeking as to why the spam wasn't blocked:
First, report spam by either forwarding the spam message to the @missedspam.com address for your domain, or by marking it as spam in the Quarantine viewer:
Reporting spam that was missed by the filter
Reporting messages as spam can help our spam techs know what is slipping through, so the rules can be upgraded. Also, once you report a message as spam, our system will let you know why the spam was missed and provide customized suggestions to help improve filtering:
Are there things I can do to improve filtering accuracy?
Following the customized suggestions as explained in the above article is your best bet. This will also reveal other possible reasons that the message was allowed through, such as:
How to prevent spam from getting around the filterThere are also other things you can do to help to reduce spam. A few examples are:
1) There is a Bulk mail filter, that will quarantine messages that are detected to be bulk senders:
How to Enable the Bulk Mail Filter
Emails considered as "bulk" have this icon by them:
However, if you use this option, and there are bulk messages that you want to receive, you will need to add the sender or domain to the Settings > Approved Senders list.
2) We also allow for Geographic filtering. So if there is a Region/country that you do not want messages from, you can block that country (quarantine), or Deep Filter the messages (delay the messages to be rescanned later with new filtering rules):
Setting up Geographic Filtering Policies
3) There is a filter to block messages with Non-English character sets & languages. You can find this in Settings > Filtering Policies:
4) There is filter to Deep filter mail from networks with high spam ratios, in Settings > Filtering Policies. The "Deep filter" will cause a message to be delayed, then rescanned with newer filtering rules:
5) There is also a way to help enforce SPF checking, by using the Deep filter mail when SPF sender authentication fails policy in Settings > Filtering Policies. If a message fails an SPF check, it will get deep filtered. You may want to ensure your domain has a proper SPF record if you use this: