diff --git a/Makefile.conf b/Makefile.conf
index fc27d98c..1b46f3e8 100644
--- a/Makefile.conf
+++ b/Makefile.conf
@@ -96,22 +96,11 @@ else ifeq ($(BUILD_FOR),Darwin)
     CC=clang 
 endif
 
-# Define platform-identification feature macros.
-ifeq ($(BUILD_FOR),Linux)
-    CPPFLAGS+= -DUNIX 
-else ifeq ($(BUILD_FOR),Darwin)
-    CPPFLAGS+= -DUNIX -DDARWIN 
-else ifeq ($(BUILD_FOR),OS/390)
-    CPPFLAGS+= -DZOS 
-    # z/OS, the way we use it, is a flavour of Unix.
-    CPPFLAGS+= -DUNIX 
-endif
-
 # Add platform-specific compilation options.
 ifeq ($(BUILD_FOR),OS/390)
     CFLAGS= -Wno-gnu-include-next -qnosearch -I$(HERE) -I.
     CFLAGS+= -I/usr/include
-    CFLAGS+= -DZOS -D_XOPEN_SOURCE=600 -D_POSIX_C_SOURCE=200112L -qlanglvl=NOLIBEXT
+    CFLAGS+= -D_XOPEN_SOURCE=600 -D_POSIX_C_SOURCE=200112L -qlanglvl=NOLIBEXT
 else ifeq ($(BUILD_FOR),Linux)
     CFLAGS+=-fPIC -fstrict-aliasing -fdiagnostics-color=auto
 else ifeq ($(BUILD_FOR),Darwin)
@@ -189,7 +178,7 @@ endif
 ifeq ($(BUILD_FOR),OS/390)
     CXXFLAGS+= -fdiagnostics-color=auto -Wno-gnu-include-next
     CXXFLAGS+= -qnosearch -I$(HERE) -I.  -I../src -I../asn.1 $(ETPAN_INC) -I/usr/lpp/java/J8.0_64/include -I/usr/include
-    CXXFLAGS+= -DZOS -D_XOPEN_SOURCE=600 -D_POSIX_C_SOURCE=200112L
+    CXXFLAGS+= -D_XOPEN_SOURCE=600 -D_POSIX_C_SOURCE=200112L
     ifdef WARN
         CXXFLAGS+=
     else
diff --git a/src/etpan_mime.c b/src/etpan_mime.c
index 755336b2..d54eb28f 100644
--- a/src/etpan_mime.c
+++ b/src/etpan_mime.c
@@ -566,7 +566,7 @@ struct mailimf_date_time * timestamp_to_etpantime(const timestamp *ts)
     result->dt_day = ts->tm_mday;
     result->dt_month = ts->tm_mon + 1;
     result->dt_year = ts->tm_year + 1900;
-#ifndef ZOS
+#ifndef __MVS__
     result->dt_zone = (int) (ts->tm_gmtoff / 36L);
 #endif
     return result;
@@ -587,14 +587,14 @@ timestamp * etpantime_to_timestamp(const struct mailimf_date_time *et)
     result->tm_mday = et->dt_day;
     result->tm_mon = et->dt_month - 1;
     result->tm_year = et->dt_year - 1900;
-#ifndef ZOS
+#ifndef __MVS__
     result->tm_gmtoff = 36L * (long) et->dt_zone;
 #endif
 
     // Normalize to UTC and then forget the offset.
     time_t t = timegm_with_gmtoff(result);
     gmtime_r(&t, result);
-#ifndef ZOS
+#ifndef __MVS__
     result->tm_gmtoff = 0;
 #endif
     return result;
diff --git a/src/pEp_internal.h b/src/pEp_internal.h
index 38ff0c22..a5fb82e7 100644
--- a/src/pEp_internal.h
+++ b/src/pEp_internal.h
@@ -75,7 +75,7 @@
 #define LOCAL_DB windoze_local_db()
 #define SYSTEM_DB windoze_system_db()
 #else // UNIX
-#ifndef ZOS
+#ifndef __MVS__
 #define _POSIX_C_SOURCE 200809L
 #endif
 #include <dlfcn.h>
diff --git a/src/platform_unix.c b/src/platform_unix.c
index 57edbba2..68b0c30b 100644
--- a/src/platform_unix.c
+++ b/src/platform_unix.c
@@ -4,7 +4,7 @@
 // This file is under GNU General Public License 3.0
 // see LICENSE.txt
 
-#ifndef ZOS
+#ifndef __MVS__
 #define _POSIX_C_SOURCE 200809L
 #endif
 
@@ -88,7 +88,7 @@ void *alloca(unsigned long x)
 }
 #endif
 
-#if defined(ANDROID) || defined(ZOS)
+#if defined(ANDROID) || defined(__MVS__)
 /* FIXME : timegm will miss when linking for x86_64 on android, when supported */
 #ifndef __LP64__ 
 time_t timegm(struct tm* const t) {
@@ -165,7 +165,7 @@ static char *_android_system_db(void)
 }
 #endif
 
-#ifdef ZOS
+#ifdef __MVS__
 char * e2as(const char * str)
 {
     char *ret = (char *)malloc(strlen(str));
diff --git a/src/platform_unix.h b/src/platform_unix.h
index 85080a18..c962d16a 100644
--- a/src/platform_unix.h
+++ b/src/platform_unix.h
@@ -7,7 +7,7 @@
 #ifndef PLATFORM_UNIX_H
 #define PLATFORM_UNIX_H
 
-#if !defined(__APPLE__) && !defined(ZOS)
+#if !defined(__APPLE__) && !defined(__MVS__)
 #define _POSIX_C_SOURCE 200809L
 #endif
 
@@ -15,17 +15,17 @@
 #include <unistd.h>
 #include <strings.h>
 #include <sys/select.h>
-#ifndef ZOS
+#ifndef __MVS__
 #include <sys/param.h>
 #endif
 #include <regex.h>
 
-#ifdef ZOS
+#ifdef __MVS__
 char * e2as(const char * str);
 char * as2e(const char * str);
 #endif
 
-#if defined(ANDROID) || defined(ZOS)
+#if defined(ANDROID) || defined(__MVS__)
 typedef unsigned char uuid_t[16];
 #else
 #include <uuid/uuid.h>
@@ -168,7 +168,7 @@ typedef char uuid_string_t[37];
 // on *nix, uuid_t is an array and already implements pointer semantics
 #define UUID uuid_t
 
-#if defined(ANDROID) || defined(ZOS)
+#if defined(ANDROID) || defined(__MVS__)
 typedef char pEpUUID[16];
 void uuid_generate_random(pEpUUID out);
 void uuid_unparse_upper(pEpUUID uu, uuid_string_t out);
diff --git a/src/platform_zos.c b/src/platform_zos.c
index 32bc3c6a..0efb6c40 100644
--- a/src/platform_zos.c
+++ b/src/platform_zos.c
@@ -4,7 +4,7 @@
 /* Include the entire useful part of this compilation unit within a CPP
    conditional, so that the entire file expands to nothing if this is not
    the platform it is used on. */
-#ifdef ZOS
+#ifdef __MVS__
 
 #include "pEpEngine.h"
 #include "platform_zos.h"
@@ -12,4 +12,4 @@
 
 /* Nothing useful here at this time. */
 
-#endif // #ifdef ZOS
+#endif // #ifdef __MVS__
diff --git a/src/platform_zos.h b/src/platform_zos.h
index 28f82f41..68ed599d 100644
--- a/src/platform_zos.h
+++ b/src/platform_zos.h
@@ -7,7 +7,7 @@
 #ifndef PEP_PLATFORM_ZOS_H
 #define PEP_PLATFORM_ZOS_H
 
-#if ! defined (ZOS)
+#if ! defined (__MVS__)
 # error "this header should only ever be included on z/OS"
 #endif
 
diff --git a/src/timestamp.c b/src/timestamp.c
index d5ac322b..79ba653e 100644
--- a/src/timestamp.c
+++ b/src/timestamp.c
@@ -22,7 +22,7 @@ DYNAMIC_API time_t timegm_with_gmtoff(const timestamp* ts)
         _ts = timestamp_dup(ts);
 
         if (_ts) {
-#if defined(ZOS)
+#if defined(__MVS__)
             const time_t raw_time = mktime(_ts);
             if (raw_time != -1) {
                 retval = raw_time;