What is a “Denial-of-Service” or DoS spam attack?
Spammers sometimes send a flood of traffic that overwhelms an email server. The result is sluggish email delivery, delaying legitimate messages from reaching their intended recipients on your network. This sluggish effect is compounded if your mail server queries a free DNS blacklist server such as Spamhaus and Spamcop. These servers are usually distant and response times to each query may add several hundred milliseconds. Also, these servers may slow down their response time if too many queries originate from your mail server in a short period. Having these IP sessions active during a denial-of-service attack will quickly exhaust a machine’s network resources and your mail server may become unable to receive inbound connection attempts from legitimate sources. In extreme cases, a mail server can crash when system resources are depleted.
