smtp-sink listens on the named host (or address) and port. It takes SMTP messages from the network and throws them away. The purpose is to measure client performance, not protocol compliance.
Connections can be accepted on IPv4 or IPv6 endpoints, or on UNIX-domain sockets. IPv4 and IPv6 are the default. This program is the complement of the smtp-source(1) program.
Described as a "black-hole" device for e-mails. Used mostly for testing purposes, it's a destination point to an MTA relay scheme.