Srizbi trojan is extremely dangerous. It is used on botnets to deliver spam and analyze database of email addresses.

Srizbi functions secretly in a background; this makes the trojan even more harmful, because its activities can’t be seen by the naked eye. Srizbi can only be detected by reputable security tools. The trojan distributes large amounts of spam messages. It also investigates delivery reports and replies to determine whether addresses are in use.

Source: http://www.spywarevoid.com/srizbi.html