Why did I receive this spam?

The spam filter will not catch all junk messages, but it should catch the vast majority of them -- 95% or more, with the exact results dependent on your configuration and on your message traffic. If you are experiencing a substantial amount of uncaught spam, there are several possible explanations:

1. Not all of the valid email addresses for a domain have been entered into the spam filter, and as a result some messages are being passed through unfiltered. By default, we only filter messages to known valid addresses, i.e. email addresses that are listed in this control panel. Any missing addresses (often alias addresses) do not receive filtering. If this is the case, you can simply add the missing addresses -- either manually or by setting up an automated process by which we automatically retrieve the valid email addresses for your domain.
2. Spammers are sending directly to your mail server, in order to circumvent our filtering. The solution is to configure your mail server and/or firewall to only accept inbound messages from our networks, which ensures that spammers cannot circumvent the filter.
3. Spammers are sending the messages from an address or domain that you have whitelisted. Make sure that you have not whitelisted your own domain (since spammers frequently forge the recipient's domain as the alleged sender of their messages in an attempt to circumvent spam filters). We also recommend against whitelisting well-known domains (Yahoo, Microsoft, etc.) as those domains are also frequently the target of spammers.
4. We are scanning the messages in question but are simply not identifying them as spam. The spam filter should catch about 98% of junk mail. If your detection rate seems significantly lower than that, there may be a problem that requires attention, most likely one or more of the items mentioned above.

If you have already addressed the items above, and are still seeing a significant percentage of spam not being caught, you may want to increase the aggressiveness of the spam filter. You may also want to try enabling greylisting for your domain, which can introduce some delays the first time you receive a message from a given sender but which can also be helpful in further reducing spam.