diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 97d5d8a..b519491 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -51,6 +51,10 @@
  * Harmless on other platforms. */
 #define _POSIX_C_SOURCE 200112L
 
+#if defined(__NetBSD__)
+#define _XOPEN_SOURCE 600
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else