diff --git a/src/.env.example b/src/.env.example
--- a/src/.env.example
+++ b/src/.env.example
@@ -16,6 +16,9 @@
 APP_WITH_RESELLER=1
 APP_WITH_SERVICES=1
 
+SIGNUP_LIMIT_EMAIL=0
+SIGNUP_LIMIT_IP=0
+
 ASSET_URL=http://127.0.0.1:8000
 
 WEBMAIL_URL=/apps
diff --git a/src/app/Rules/ExternalEmail.php b/src/app/Rules/ExternalEmail.php
--- a/src/app/Rules/ExternalEmail.php
+++ b/src/app/Rules/ExternalEmail.php
@@ -37,6 +37,28 @@
             return false;
         }
 
+        // don't allow multiple open registrations against the same email address
+        if (env('SIGNUP_LIMIT_EMAIL', 0) > 0) {
+            $signups = \App\SignupCode::where('email', $email)
+                ->whereDate('expires_at', '>', \Carbon\Carbon::now());
+
+            if ($signups->count() >= env('SIGNUP_LIMIT_EMAIL')) {
+                $this->message = \trans('validation.emailinvalid');
+                return false;
+            }
+        }
+
+        // don't allow multiple open registrations against the same source ip address
+        if (env('SIGNUP_LIMIT_IP', 0) > 0) {
+            $signups = \App\SignupCode::where("ip_address", request()->ip())
+                ->whereDate('expires_at', '>', \Carbon\Carbon::now());
+
+            if ($signups->count() >= env('SIGNUP_LIMIT_IP')) {
+                $this->message = \trans('validation.emailinvalid');
+                return false;
+            }
+        }
+
         return true;
     }
 
diff --git a/src/app/SignupCode.php b/src/app/SignupCode.php
--- a/src/app/SignupCode.php
+++ b/src/app/SignupCode.php
@@ -99,8 +99,18 @@
      */
     public static function generateShortCode(): string
     {
-        $code_length = env('SIGNUP_CODE_LENGTH', self::SHORTCODE_LENGTH);
+        $codeLength = env('SIGNUP_CODE_LENGTH', self::SHORTCODE_LENGTH);
 
-        return \App\Utils::randStr($code_length);
+        $allegedlyUnique = \App\Utils::randStr($codeLength);
+
+        while ($code = $this->where('short_code', $allegedlyUnique)->first()) {
+            if ($code->isExpired()) {
+                break;
+            }
+
+            $allegedlyUnique = \App\Utils::randStr($codeLength);
+        }
+
+        return $allegedlyUnique;
     }
 }