If you allow incoming mail over an IPC connection, you should have a daemon running. This should be set by your /etc/rc file using the -bd flag. The -bd flag and the -q flag may be combined in one call:
An alternative approach is to invoke sendmail from inetd(8) (use the -bs flag to ask sendmail to speak SMTP on its standard input and output). This works and allows you to wrap sendmail in a TCP wrapper program, but may be a bit slower since the configuration file has to be re-read on every message that comes in. If you do this, you still need to have a sendmail running to flush the queue: