diff --git a/modoboa/admin/tests/test_mapfiles.py b/modoboa/admin/tests/test_mapfiles.py
index a6fa41ade..79f424176 100644
--- a/modoboa/admin/tests/test_mapfiles.py
+++ b/modoboa/admin/tests/test_mapfiles.py
@@ -1,7 +1,12 @@
 """Test map files generation."""
 
+import os
+
+from django.conf import settings
+from django.core.management import call_command
 from django.test import TestCase
 
+from modoboa.core.utils import parse_map_file
 from modoboa.lib.test_utils import MapFilesTestCaseMixin
 
 
@@ -14,3 +19,38 @@ class MapFilesTestCase(MapFilesTestCaseMixin, TestCase):
         "sql-maintain.cf", "sql-sender-login-mailboxes.cf",
         "sql-sender-login-mailboxes-extra.cf", "sql-sender-login-aliases.cf"
     ]
+
+    def test_map_upgrade(self):
+        """Check that map content is used."""
+        dburl = "postgres://user:password@localhost/testdb"
+        call_command(
+            "generate_postfix_maps",
+            "--dburl", dburl, "--destdir", self.workdir)
+
+        # Now upgrade files. Map credentials should be preserved.
+        call_command("generate_postfix_maps", "--destdir", self.workdir)
+        for f in self.MAP_FILES:
+            mapcontent = parse_map_file(os.path.join(self.workdir, f))
+            self.assertEqual(mapcontent["user"], "user")
+            self.assertEqual(mapcontent["password"], "password")
+            self.assertEqual(mapcontent["dbname"], "testdb")
+
+        # Now force overwrite, credentials should be different
+        call_command(
+            "generate_postfix_maps", "--destdir", self.workdir,
+            "--force-overwrite")
+        dbsettings = settings.DATABASES["default"]
+        for f in self.MAP_FILES:
+            mapcontent = parse_map_file(os.path.join(self.workdir, f))
+            self.assertEqual(mapcontent["user"], dbsettings["USER"])
+            self.assertEqual(mapcontent["password"], dbsettings["PASSWORD"])
+            self.assertEqual(mapcontent["dbname"], dbsettings["NAME"])
+
+        # Now modify a file manually
+        path = os.path.join(self.workdir, "sql-domains.cf")
+        with open(path, "a") as fp:
+            fp.write("pouet")
+        call_command("generate_postfix_maps", "--destdir", self.workdir)
+        with open(path) as fp:
+            content = fp.read()
+        self.assertIn("pouet", content)
diff --git a/modoboa/core/management/commands/generate_postfix_maps.py b/modoboa/core/management/commands/generate_postfix_maps.py
index ae29af77f..bb3e915ab 100644
--- a/modoboa/core/management/commands/generate_postfix_maps.py
+++ b/modoboa/core/management/commands/generate_postfix_maps.py
@@ -1,5 +1,7 @@
 """Management command to generate/update postfix map files."""
 
+import copy
+import hashlib
 import os
 import sys
 
@@ -11,6 +13,7 @@ from django.utils import timezone
 import dj_database_url
 
 from ... import signals
+from ... import utils
 
 MAP_FILE_TEMPLATE = """# This file was generated on {{ date }} by running:
 # {{ commandline }}
@@ -30,6 +33,23 @@ class Command(BaseCommand):
         parser.add_argument(
             "--destdir", default=".",
             help="Directory where files will be created")
+        parser.add_argument(
+            "--force-overwrite", action="store_true", default=False,
+            help="Force overwrite of existing map files")
+
+    def __load_checksums(self, destdir):
+        """Load existing checksums if possible."""
+        self.__checksums_file = os.path.join(
+            destdir, "modoboa-postfix-maps.chk")
+        self.__checksums = {}
+        if not os.path.exists(self.__checksums_file):
+            return
+        with open(self.__checksums_file) as fp:
+            for line in fp:
+                fname, dbtype, checksum = line.split(":")
+                self.__checksums[fname.strip()] = {
+                    "dbtype": dbtype, "checksum": checksum.strip()
+                }
 
     def __register_map_files(self):
         """Load specified applications."""
@@ -39,6 +59,18 @@ class Command(BaseCommand):
             mapfiles += response[1]
         return mapfiles
 
+    def __check_file(self, path):
+        """Check if map file has been modified."""
+        fname = os.path.basename(path)
+        condition = (
+            not self.__checksums or
+            fname not in self.__checksums)
+        if condition:
+            return True
+        with open(path) as fp:
+            checksum = hashlib.md5(fp.read()).hexdigest()
+        return checksum == self.__checksums[fname]["checksum"]
+
     def get_template(self, dbtype):
         """Return map file template."""
         tplcontent = MAP_FILE_TEMPLATE
@@ -80,8 +112,23 @@ query = {{ query|safe }}
         }
         return context
 
-    def __render_map_file(self, mapobject, destdir, context):
+    def __render_map_file(
+            self, mapobject, destdir, context, force_overwrite=False):
         """Render a map file."""
+        fullpath = os.path.join(destdir, mapobject.filename)
+        if os.path.exists(fullpath) and not force_overwrite:
+            if not self.__check_file(fullpath):
+                print(
+                    "Cannot upgrade '{}' map because it has been modified."
+                    .format(mapobject.filename))
+                return self.__checksums[mapobject.filename]
+            mapcontent = utils.parse_map_file(fullpath)
+            context = copy.deepcopy(context)
+            context["dbtype"] = self.__checksums[mapobject.filename]["dbtype"]
+            context["dbuser"] = mapcontent["user"]
+            context["dbpass"] = mapcontent["password"]
+            context["dbname"] = mapcontent["dbname"]
+            context["dbhost"] = mapcontent["hosts"]
         content = self.get_template(context["dbtype"]).render(
             Context(
                 dict(context.items(),
@@ -90,7 +137,8 @@ query = {{ query|safe }}
         )
         fullpath = os.path.join(destdir, mapobject.filename)
         with open(fullpath, "w") as fp:
-            fp.write("{}\n".format(content))
+            fp.write(content)
+        return hashlib.md5(content).hexdigest()
 
     def handle(self, *args, **options):
         """Command entry point."""
@@ -100,6 +148,15 @@ query = {{ query|safe }}
             os.mkdir(destdir)
         except OSError:
             pass
+        self.__load_checksums(destdir)
         context = self.get_template_context(options)
+        checksums = {}
         for mapobject in mapfiles:
-            self.__render_map_file(mapobject, destdir, context)
+            checksum = self.__render_map_file(
+                mapobject, destdir, context,
+                force_overwrite=options["force_overwrite"])
+            checksums[mapobject.filename] = checksum
+        with open(self.__checksums_file, "w") as fp:
+            for fname, checksum in checksums.items():
+                fp.write("{}:{}:{}\n".format(
+                    fname, context["dbtype"], checksum))
diff --git a/modoboa/core/utils.py b/modoboa/core/utils.py
index 6eb0afa36..371e5b33a 100644
--- a/modoboa/core/utils.py
+++ b/modoboa/core/utils.py
@@ -11,6 +11,18 @@ from modoboa.lib.api_client import ModoAPIClient
 from . import models
 
 
+def parse_map_file(path):
+    """Parse a postfix map file and return values."""
+    content = {}
+    with open(path) as fp:
+        for line in fp:
+            if not line or line.startswith("#"):
+                continue
+            name, value = line.split("=", 1)
+            content[name.strip()] = value.strip()
+    return content
+
+
 def check_for_updates(request):
     """Check if a new version of Modoboa is available."""
     local_config = models.LocalConfig.objects.first()