diff --git a/src/applications/config/check/PhabricatorExtraConfigSetupCheck.php b/src/applications/config/check/PhabricatorExtraConfigSetupCheck.php
--- a/src/applications/config/check/PhabricatorExtraConfigSetupCheck.php
+++ b/src/applications/config/check/PhabricatorExtraConfigSetupCheck.php
@@ -380,6 +380,11 @@
         'Resource deflation is now managed automatically.'),
       'celerity.minify' => pht(
         'Resource minificatino is now managed automatically.'),
+
+      'metamta.domain' => pht(
+        'Mail thread IDs are now generated automatically.'),
+      'metamta.placeholder-to-recipient' => pht(
+        'Placeholder recipients are now generated automatically.'),
     );
 
     return $ancient_config;
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
@@ -139,13 +139,6 @@
 great. A number of other mailers are available (e.g., SES, SendGrid, SMTP,
 custom mailers). This option is deprecated in favor of 'cluster.mailers'.
 EODOC
-));
-
-    $placeholder_description = $this->deformat(pht(<<<EODOC
-When sending a message that has no To recipient (i.e. all recipients are CC'd),
-set the To field to the following value. If no value is set, messages with no
-To will have their CCs upgraded to To.
-EODOC
 ));
 
     $public_replies_description = $this->deformat(pht(<<<EODOC
@@ -203,11 +196,6 @@
         'string',
         'noreply@phabricator.example.com')
         ->setDescription(pht('Default "From" address.')),
-      $this->newOption(
-        'metamta.domain',
-        'string',
-        'phabricator.example.com')
-        ->setDescription(pht('Domain used to generate Message-IDs.')),
       $this->newOption(
         'metamta.one-mail-per-recipient',
         'bool',
@@ -265,9 +253,6 @@
           ))
         ->setSummary(pht('Trust "Reply-To" headers for authentication.'))
         ->setDescription($reply_to_description),
-      $this->newOption('metamta.placeholder-to-recipient', 'string', null)
-        ->setSummary(pht('Placeholder for mail with only CCs.'))
-        ->setDescription($placeholder_description),
       $this->newOption('metamta.public-replies', 'bool', false)
         ->setBoolOptions(
           array(
diff --git a/src/applications/metamta/constants/MetaMTAReceivedMailStatus.php b/src/applications/metamta/constants/MetaMTAReceivedMailStatus.php
--- a/src/applications/metamta/constants/MetaMTAReceivedMailStatus.php
+++ b/src/applications/metamta/constants/MetaMTAReceivedMailStatus.php
@@ -17,6 +17,7 @@
   const STATUS_UNHANDLED_EXCEPTION  = 'err:exception';
   const STATUS_EMPTY                = 'err:empty';
   const STATUS_EMPTY_IGNORED        = 'err:empty-ignored';
+  const STATUS_VOID_RECIPIENT       = 'err:void';
 
   public static function getHumanReadableName($status) {
     $map = array(
@@ -34,6 +35,7 @@
       self::STATUS_UNHANDLED_EXCEPTION => pht('Unhandled Exception'),
       self::STATUS_EMPTY => pht('Empty Mail'),
       self::STATUS_EMPTY_IGNORED => pht('Ignored Empty Mail'),
+      self::STATUS_VOID_RECIPIENT => pht('Void Recipient'),
     );
 
     return idx($map, $status, pht('Processing Exception'));
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAMail.php
@@ -857,7 +857,7 @@
           // aren't in the form "<string@domain.tld>"; this is also required
           // by RFC 2822, although some clients are more liberal in what they
           // accept.
-          $domain = PhabricatorEnv::getEnvConfig('metamta.domain');
+          $domain = $this->newMailDomain();
           $value = '<'.$value.'@'.$domain.'>';
 
           if ($is_first && $mailer->supportsMessageIDHeader()) {
@@ -1017,18 +1017,13 @@
       return null;
     }
 
-    // Some mailers require a valid "To:" in order to deliver mail. If we
-    // don't have any "To:", try to fill it in with a placeholder "To:".
-    // If that also fails, move the "Cc:" line to "To:".
+    // Some mailers require a valid "To:" in order to deliver mail. If we don't
+    // have any "To:", fill it in with a placeholder "To:". This allows client
+    // rules based on whether the recipient is in "To:" or "CC:" to continue
+    // behaving in the same way.
     if (!$add_to) {
-      $placeholder_key = 'metamta.placeholder-to-recipient';
-      $placeholder = PhabricatorEnv::getEnvConfig($placeholder_key);
-      if ($placeholder !== null) {
-        $add_to = array($placeholder);
-      } else {
-        $add_to = $add_cc;
-        $add_cc = array();
-      }
+      $void_recipient = $this->newVoidEmailAddress();
+      $add_to = array($void_recipient->getAddress());
     }
 
     $add_to = array_unique($add_to);
@@ -1467,6 +1462,19 @@
     return '/mail/detail/'.$this->getID().'/';
   }
 
+  private function newMailDomain() {
+    $install_uri = PhabricatorEnv::getURI('/');
+    $install_uri = new PhutilURI($install_uri);
+
+    return $install_uri->getDomain();
+  }
+
+  public function newVoidEmailAddress() {
+    $domain = $this->newMailDomain();
+    $address = "void-recipient@{$domain}";
+    return new PhutilEmailAddress($address);
+  }
+
 
 /* -(  Routing  )------------------------------------------------------------ */
 
diff --git a/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php b/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php
--- a/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php
+++ b/src/applications/metamta/storage/PhabricatorMetaMTAReceivedMail.php
@@ -109,6 +109,7 @@
     try {
       $this->dropMailFromPhabricator();
       $this->dropMailAlreadyReceived();
+      $this->dropMailToVoidRecipient();
 
       $receiver = $this->loadReceiver();
       $sender = $receiver->loadSender($this);
@@ -260,6 +261,36 @@
       $message);
   }
 
+  private function dropMailToVoidRecipient() {
+    $void_address = id(new PhabricatorMetaMTAMail())
+      ->newVoidEmailAddress();
+
+    $void_recipient = null;
+    foreach ($this->getToAddresses() as $raw_address) {
+      try {
+        $address = new PhutilEmailAddress($raw_address);
+      } catch (Exception $ex) {
+        continue;
+      }
+
+      if ($address->getAddress() == $void_address->getAddress()) {
+        $void_recipient = $address;
+        break;
+      }
+    }
+
+    if (!$void_recipient) {
+      return;
+    }
+
+    throw new PhabricatorMetaMTAReceivedMailProcessingException(
+      MetaMTAReceivedMailStatus::STATUS_VOID_RECIPIENT,
+      pht(
+        'Ignoring email to void recipient ("%s"). This is a placeholder '.
+        'recipient Phabricator generates when a message only has "CC" '.
+        'addresses.',
+        $void_recipient->getAddress()));
+  }
 
   /**
    * Load a concrete instance of the @{class:PhabricatorMailReceiver} which
diff --git a/src/docs/user/configuration/configuring_outbound_email.diviner b/src/docs/user/configuration/configuring_outbound_email.diviner
--- a/src/docs/user/configuration/configuring_outbound_email.diviner
+++ b/src/docs/user/configuration/configuring_outbound_email.diviner
@@ -45,7 +45,6 @@
   - **metamta.default-address** determines where mail is sent "From" by
     default. If your domain is `example.org`, set this to something like
     `noreply@example.org`.
-  - **metamta.domain** should be set to your domain, e.g. `example.org`.
   - **metamta.can-send-as-user** should be left as `false` in most cases,
     but see the documentation for details.