Gross is a greylisting server. The features that make gross stand out from other greylisters are:

- it's blazingly fast
- it's amazingly resource efficient
- it can be configured to query DNSBL databases, and enforce greylisting only for hosts that are found on those databases
- it can block hosts that match multiple DNSBL's
- it can be replicated and run parallel on two servers
- it supports Sun Java System Messaging Server, Postfix, Sendmail and Exim.