+/* for more specific version constraints, look at __FreeBSD_version
+ * from <sys/param.h> */
+
+/* When using DKIM, setting OS_SENDFILE can increase
+performance on outgoing mail a bit. */
+
+#define OS_SENDFILE
+extern ssize_t os_sendfile(int, int, off_t *, size_t);
+
+
+/*******************/
+
+/* TCP_FASTOPEN support. There does not seems to be a
+MSG_FASTOPEN defined yet... */
+#define EXIM_TFO_PROBE
+
+#include <netinet/tcp.h> /* for TCP_FASTOPEN */
+#include <sys/socket.h> /* for MSG_FASTOPEN */
+#if defined(TCP_FASTOPEN) && !defined(MSG_FASTOPEN)
+# define MSG_FASTOPEN 0x20000000
+#endif
+
+/* for TCP state-variable values, for TFO logging */
+#include <netinet/tcp_fsm.h>
+#define TCP_SYN_RECV TCPS_SYN_RECEIVED
+
+/*******************/