I get tired of people trying to sell their spam filtering and all they do is turn up every filter so that all email is marked/quarantined as spam and they expect you to whitelist everything you want to get.
So what is whitelisting? Well it is a list of accepted items, in this case email address or domains. When whitelisted an emails skip the filtering so that they are delivered. If I wanted to make sure that any email from z3r010v3@gmail.com would get delivered to me I could whitelist the whole email or I could whitelist @gmail.com and then it would always get to me. This is not the way to do it, I believe whitelists should be a last resort.
With computers being infected with viruses that allow spammers to send emails from them, and email servers configured wrong allowing for open relays there is always a chance a domain you have whitelisted will spam you. To top it off most whitelist use the "From" address and this is easy to bypass. I had a client for example that whitelisted *@*.gov so that all emails from anything ending in .gov would get through. That is fine till you have a person complaining at the clients office because they are getting spam from not.a.real.host.gov. Then you have spammers that are smart and figure out what email address to use based on the company. For example here in Alabama lawyers always want to whitelist alacourt.gov, yea didn't take spammers long to figure out they could spam any law office in Alabama if they just used the alacourt domain for their email address.
The same that goes here for whitelisting also applies to blacklist. I have had clients inadvertently blacklist everyone at aol.com or gmail.com. So the same goes here for blacklisting.
If you do not trust your rules and filters work at them more. Don't just start whitelisting every tom, dick, and harry.com
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