diff --git a/src/applications/config/option/PhabricatorCoreConfigOptions.php b/src/applications/config/option/PhabricatorCoreConfigOptions.php
--- a/src/applications/config/option/PhabricatorCoreConfigOptions.php
+++ b/src/applications/config/option/PhabricatorCoreConfigOptions.php
@@ -90,9 +90,10 @@
         ->addExample('America/Boise', pht('US Mountain (MDT)'))
         ->addExample('America/Los_Angeles', pht('US West (PDT)')),
       $this->newOption('phabricator.cookie-prefix', 'string', null)
+        ->setLocked(true)
         ->setSummary(
           pht('Set a string Phabricator should use to prefix '.
-              'cookie names'))
+              'cookie names.'))
         ->setDescription(
           pht(
             'Cookies set for x.com are also sent for y.x.com. Assuming '.
diff --git a/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php b/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php
--- a/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php
+++ b/src/applications/config/option/PhabricatorMetaMTAConfigOptions.php
@@ -244,6 +244,7 @@
         'metamta.reply-handler-domain',
         'string',
         null)
+        ->setLocked(true)
         ->setDescription(pht(
           'Domain used for reply email addresses. Some applications can '.
           'override this configuration with a different domain.'))
diff --git a/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php b/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
--- a/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
+++ b/src/applications/differential/config/PhabricatorDifferentialConfigOptions.php
@@ -240,12 +240,14 @@
         'metamta.differential.reply-handler-domain',
         'string',
         null)
+        ->setLocked(true)
         ->setDescription(
           pht('Inbound email domain for Differential replies.')),
       $this->newOption(
         'metamta.differential.reply-handler',
         'class',
         'DifferentialReplyHandler')
+        ->setLocked(true)
         ->setBaseClass('PhabricatorMailReplyHandler')
         ->setDescription(pht('Alternate reply handler class.')),
       $this->newOption(
diff --git a/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php b/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php
--- a/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php
+++ b/src/applications/diffusion/config/PhabricatorDiffusionConfigOptions.php
@@ -30,6 +30,7 @@
         'metamta.diffusion.reply-handler-domain',
         'string',
         null)
+        ->setLocked(true)
         ->setDescription(
           pht(
             'See {{metamta.maniphest.reply-handler}}. This does the same '.
@@ -38,6 +39,7 @@
         'metamta.diffusion.reply-handler',
         'class',
         'PhabricatorAuditReplyHandler')
+        ->setLocked(true)
         ->setBaseClass('PhabricatorMailReplyHandler')
         ->setDescription(pht('Override mail reply handler class.')),
       $this->newOption(
diff --git a/src/applications/macro/config/PhabricatorMacroConfigOptions.php b/src/applications/macro/config/PhabricatorMacroConfigOptions.php
--- a/src/applications/macro/config/PhabricatorMacroConfigOptions.php
+++ b/src/applications/macro/config/PhabricatorMacroConfigOptions.php
@@ -22,6 +22,7 @@
   public function getOptions() {
     return array(
       $this->newOption('metamta.macro.reply-handler-domain', 'string', null)
+        ->setLocked(true)
         ->setDescription(pht(
           'As {{metamta.maniphest.reply-handler-domain}}, but affects Macro.')),
       $this->newOption('metamta.macro.subject-prefix', 'string', '[Macro]')
diff --git a/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php b/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
--- a/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
+++ b/src/applications/maniphest/config/PhabricatorManiphestConfigOptions.php
@@ -276,6 +276,7 @@
         'metamta.maniphest.reply-handler-domain',
         'string',
         null)
+        ->setLocked(true)
         ->setSummary(pht('Enable replying to tasks via email.'))
         ->setDescription(
           pht(
@@ -296,6 +297,7 @@
         'metamta.maniphest.reply-handler',
         'class',
         'ManiphestReplyHandler')
+        ->setLocked(true)
         ->setBaseClass('PhabricatorMailReplyHandler')
         ->setDescription(pht('Override reply handler class.')),
       $this->newOption(
diff --git a/src/applications/owners/config/PhabricatorOwnersConfigOptions.php b/src/applications/owners/config/PhabricatorOwnersConfigOptions.php
--- a/src/applications/owners/config/PhabricatorOwnersConfigOptions.php
+++ b/src/applications/owners/config/PhabricatorOwnersConfigOptions.php
@@ -25,6 +25,7 @@
         'metamta.package.reply-handler',
         'class',
         'OwnersPackageReplyHandler')
+        ->setLocked(true)
         ->setBaseClass('PhabricatorMailReplyHandler')
         ->setDescription(pht('Reply handler for owners mail.')),
       $this->newOption('metamta.package.subject-prefix', 'string', '[Package]')
diff --git a/src/applications/pholio/config/PhabricatorPholioConfigOptions.php b/src/applications/pholio/config/PhabricatorPholioConfigOptions.php
--- a/src/applications/pholio/config/PhabricatorPholioConfigOptions.php
+++ b/src/applications/pholio/config/PhabricatorPholioConfigOptions.php
@@ -22,6 +22,7 @@
   public function getOptions() {
     return array(
       $this->newOption('metamta.pholio.reply-handler-domain', 'string', null)
+        ->setLocked(true)
         ->setDescription(
           pht(
             'Like {{metamta.maniphest.reply-handler-domain}}, but affects '.