Index: auto.def
==================================================================
--- auto.def
+++ auto.def
@@ -513,11 +513,29 @@
     # Last resort, may be Windows
     if {[is_mingw]} {
         define-append LIBS -lwsock32
     }
 }
-cc-check-function-in-lib ns_name_uncompress resolv
+
+# The SMTP module requires special libraries and headers for MX DNS
+# record lookups and such.
+cc-check-includes arpa/nameser.h
+cc-include-needs bind/resolv.h netinet/in.h
+cc-check-includes bind/resolv.h
+cc-check-includes resolv.h
+if {    !(([cc-check-function-in-lib dn_expand resolv] ||
+           [cc-check-function-in-lib   ns_name_uncompress {bind resolv}] ||
+           [cc-check-function-in-lib __ns_name_uncompress {bind resolv}]) &&
+          ([cc-check-function-in-lib   ns_parserr {bind resolv}] ||
+           [cc-check-function-in-lib __ns_parserr {bind resolv}]) &&
+          ([cc-check-function-in-lib   res_query {bind resolv}] ||
+           [cc-check-function-in-lib __res_query {bind resolv}]))} {
+    msg-result "WARNING: SMTP feature will not be able to look up local MX."
+}
+cc-check-function-in-lib res_9_ns_initparse resolv
+
+# Other nonstandard function checks
 cc-check-functions utime
 cc-check-functions usleep
 cc-check-functions strchrnul
 cc-check-functions pledge
 cc-check-functions backtrace

Index: src/smtp.c
==================================================================
--- src/smtp.c
+++ src/smtp.c
@@ -19,15 +19,27 @@
 ** to RFC 5321.
 */
 #include "config.h"
 #include "smtp.h"
 #include <assert.h>
-#if defined(__linux__) && !defined(FOSSIL_OMIT_DNS)
+#if (HAVE_DN_EXPAND || HAVE___NS_NAME_UNCOMPRESS || HAVE_NS_NAME_UNCOMPRESS) && \
+    (HAVE_NS_PARSERR || HAVE___NS_PARSERR) && !defined(FOSSIL_OMIT_DNS)
 #  include <sys/types.h>
 #  include <netinet/in.h>
-#  include <arpa/nameser.h>
-#  include <resolv.h>
+#  if defined(HAVE_BIND_RESOLV_H)
+#    include <bind/resolv.h>
+#    include <bind/arpa/nameser_compat.h>
+#  else
+#    include <arpa/nameser.h>
+#    include <resolv.h>
+#  endif
+#  if defined(HAVENS_NAME_UNCOMPRESS) && !defined(dn_expand)
+#    define dn_expand ns_name_uncompress
+#  endif
+#  if defined(HAVE__NS_NAME_UNCOMPRESS) && !defined(dn_expand)
+#    define dn_expand __ns_name_uncompress
+#  endif
 #  define FOSSIL_UNIX_STYLE_DNS 1
 #endif
 #if defined(_WIN32) && !defined(__MINGW32__) && !defined(__MINGW64__)
 #  include <windows.h>
 #  include <windns.h>
@@ -77,12 +89,11 @@
         iBestPriority = priority;
       }
     }
   }
   if( pBest ){
-    ns_name_uncompress(aDns, aDns+nDns, pBest+2,
-                       zHostname, sizeof(zHostname));
+    dn_expand(aDns, aDns+nDns, pBest+2, zHostname, sizeof(zHostname));
     return fossil_strdup(zHostname);
   }
   return 0;
 #elif defined(FOSSIL_WINDOWS_STYLE_DNS)
   DNS_STATUS status;           /* Return status */