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sendmail stores a large amount of information about each remote system it has connected to in memory. It is now possible to preserve some of this information on disk as well, by using the HostStatusDirectory option, so that it may be shared between several invocations of sendmail. This allows mail to be queued immediately or skipped during a queue run if there has been a recent failure in connecting to a remote machine.
Additionally enabling SingleThreadDelivery has the added effect of single-threading mail delivery to a destination. This can be quite helpful if the remote machine is running an SMTP server that is easily overloaded or cannot accept more than a single connection at a time, but can cause some messages to be punted to a future queue run. It also applies to all hosts, so setting this because you have one machine on site that runs some software that is easily overrun can cause mail to other hosts to be slowed down. If this option is set, you probably want to set the MinQueueAge option as well and run the queue fairly frequently; this way jobs that are skipped because another sendmail is talking to the same host will be tried again quickly rather than being delayed for a long time.
The disk based host information is stored in a subdirectory of the mqueue directory called .hoststat (controlled by the HostStatusDirectory option). Removing this directory and its subdirectories has an effect similar to the purgestat command and is completely safe. The information in these directories can be perused with the hoststat command, which will indicate the host name, the last access, and the status of that access. An asterisk in the left most column indicates that a sendmail process currently has the host locked for mail delivery.
The disk based connection information is treated the same way as memory based connection information for the purpose of timeouts. By default, information about host failures is valid for 30 minutes. This can be adjusted with the Timeout.hoststatus option.
The connection information stored on disk may be purged at any time with the purgestat command or by invoking sendmail with the -bH switch. The connection information may be viewed with the hoststat command or by invoking sendmail with the -bh switch.