diff --git a/devenv.lock b/devenv.lock
index 6d0c9cf..643fafc 100644
--- a/devenv.lock
+++ b/devenv.lock
@@ -3,11 +3,11 @@
     "devenv": {
       "locked": {
         "dir": "src/modules",
-        "lastModified": 1784325378,
-        "narHash": "sha256-Gof6j4d43yX2qSLLp78JILke346IggDFIxTgl3ecVQE=",
+        "lastModified": 1787015266,
+        "narHash": "sha256-B4eXU1+QiZc3fFgH0pTl185xNUYNzW+1KIe9Jmtavkk=",
         "owner": "cachix",
         "repo": "devenv",
-        "rev": "5f1cf17be0fc48689bd0ecb810de6d2e06d259a1",
+        "rev": "c18de5253016e6eb97689724327f08e9dd2d4768",
         "type": "github"
       },
       "original": {
@@ -22,11 +22,11 @@
         "nixpkgs-src": "nixpkgs-src"
       },
       "locked": {
-        "lastModified": 1783345554,
-        "narHash": "sha256-LZhOm4kqjHUnwP4CNHpaqqEVcumGNd1PvOEOad8LkS0=",
+        "lastModified": 1787002832,
+        "narHash": "sha256-DGkHh88/7YLVGZ7HzSVN4YmUmR+wGKudIp6H9UsUNT0=",
         "owner": "cachix",
         "repo": "devenv-nixpkgs",
-        "rev": "6004ea8c229fe9d41b21c6f4c76bf6c2e10771dd",
+        "rev": "ee3d58d53cfcddfc0ae6fc7f04f4fe2a0c7cf0ed",
         "type": "github"
       },
       "original": {
@@ -39,11 +39,11 @@
     "nixpkgs-src": {
       "flake": false,
       "locked": {
-        "lastModified": 1783279667,
-        "narHash": "sha256-/NAkDSsve+GNM0Bt6tleJdCGfsTlK89nPjkVOzZMo0s=",
+        "lastModified": 1786858016,
+        "narHash": "sha256-Vczr2zxD0orq6N2QQe5uqOGF7TRDcQJRRuAcsdHr4kg=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "f205b5574fd0cb7da5b702a2da51507b7f4fdd1b",
+        "rev": "54ba4bcec4043e72a4006d825e0d7aff5562008f",
         "type": "github"
       },
       "original": {
diff --git a/devenv.nix b/devenv.nix
index 33faf76..a7ba381 100644
--- a/devenv.nix
+++ b/devenv.nix
@@ -38,6 +38,10 @@
         alias ${config.env.DEVENV_ROOT}/themes/;
       }
 
+      location /mu-plugins/ {
+        alias ${config.env.DEVENV_ROOT}/mu-plugins/;
+      }
+
       location ~ \.php$ {
         try_files $uri =404;
         include ${pkgs.nginx}/conf/fastcgi.conf;
@@ -71,6 +75,8 @@
     ];
   };
 
+  services.mailhog.enable = true;
+
   services.mysql.enable = true;
   services.mysql.ensureUsers = [
     {
diff --git a/mu-plugins/mailhog.php b/mu-plugins/mailhog.php
new file mode 100644
index 0000000..e0f4b57
--- /dev/null
+++ b/mu-plugins/mailhog.php
@@ -0,0 +1,21 @@
+<?php
+/**
+ * Route all outgoing mail through the local mailhog service (see devenv.nix)
+ * instead of actually sending it. View caught mail at http://localhost:8025.
+ */
+
+add_action(
+	'phpmailer_init',
+	function ( $phpmailer ) {
+		$phpmailer->isSMTP();
+		$phpmailer->Host       = '127.0.0.1';
+		$phpmailer->Port       = 1025;
+		$phpmailer->SMTPAuth   = false;
+		$phpmailer->SMTPAutoTLS = false;
+	}
+);
+
+// WordPress defaults the From address to "wordpress@localhost" in this
+// environment, which PHPMailer rejects as invalid (no dot in the domain),
+// causing wp_mail() to fail before it ever reaches mailhog.
+add_filter( 'wp_mail_from', fn() => 'wordpress@wp-devenv.test' );
diff --git a/wp-config.php b/wp-config.php
index ed2f607..1dc082a 100644
--- a/wp-config.php
+++ b/wp-config.php
@@ -10,6 +10,10 @@
 define( 'WP_PLUGIN_DIR', __DIR__ . '/plugins' );
 define( 'WP_PLUGIN_URL', 'http://localhost:8080/plugins' );
 
+// ** Must-use plugins also live in the repo root ** //
+define( 'WPMU_PLUGIN_DIR', __DIR__ . '/mu-plugins' );
+define( 'WPMU_PLUGIN_URL', 'http://localhost:8080/mu-plugins' );
+
 // ** Database settings (matches services.mysql in devenv.nix) ** //
 define( 'DB_NAME', 'wp' );
 define( 'DB_USER', 'wp' );