It's a common complaint - "We have antispam software, so why do we still receive spam ?".

It's simple really - antispam software is mostly playing catch up most of the time. In spite of regular updates to signature files and heuristics rules, the simple fact of the matter is, antispam software is no match to a person's ability to read and interpret messages no matter how badly a cleverly misspelt word or paragraph looks. Throw in a mixture of text and graphics to form a spammer's message and the antispam software will likely miss it. Setting your antispam software too sensitive and you will likely end up spending more time taking out legitimate mails from the quarantine (assuming you actually quarantine that is !) than actually reading them.