When processing the queue, sendmail will try to keep the last few open connections open to avoid startup and shutdown costs. This only applies to IPC connections.
When trying to open a connection the cache is first searched. If an open connection is found, it is probed to see if it is still active by sending a command. It is not an error if this fails; instead, the connection is closed and reopened.
Two parameters control the connection cache. The ConnectionCacheSize ( k) option defines the number of simultaneous open connections that will be permitted. If it is set to zero, connections will be closed as quickly as possible. The default is one. This should be set as appropriate for your system size; it will limit the amount of system resources that sendmail will use during queue runs. Never set this higher than 4.
The ConnectionCacheTimeout ( K) option specifies the maximum time that any cached connection will be permitted to idle. When the idle time exceeds this value the connection is closed. This number should be small (under ten minutes) to prevent you from grabbing too many resources from other hosts. The default is five minutes.