diff --git a/config/site_settings.yml b/config/site_settings.yml
index 547f7654c42..1f6c8f0a9f7 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -705,6 +705,7 @@ email:
     validator: "ReplyByEmailEnabledValidator"
   reply_by_email_address:
     default: ''
+    validator: "ReplyByEmailAddressValidator"
   alternative_reply_by_email_addresses:
     default: ''
     validator: "AlternativeReplyByEmailAddressesValidator"
diff --git a/lib/validators/reply_by_email_address_validator.rb b/lib/validators/reply_by_email_address_validator.rb
index 3c3b5698f89..ff668bf8582 100644
--- a/lib/validators/reply_by_email_address_validator.rb
+++ b/lib/validators/reply_by_email_address_validator.rb
@@ -4,11 +4,14 @@ class ReplyByEmailAddressValidator
   end
 
   def valid_value?(val)
-    return true if val.blank?
+    return true  if val.blank?
+    return false if val["@"].nil?
 
-    !!val["@"] &&
-    !!val["%{reply_key}"] &&
-    val.gsub(/\+?%{reply_key}/, "") != SiteSetting.notification_email
+    if SiteSetting.find_related_post_with_key
+      !!val["%{reply_key}"] && val.sub(/\+?%{reply_key}/, "") != SiteSetting.notification_email
+    else
+      val != SiteSetting.notification_email
+    end
   end
 
   def error_message
diff --git a/spec/components/validators/reply_by_email_address_validator_spec.rb b/spec/components/validators/reply_by_email_address_validator_spec.rb
index ac580b7f016..cf7a889a4a7 100644
--- a/spec/components/validators/reply_by_email_address_validator_spec.rb
+++ b/spec/components/validators/reply_by_email_address_validator_spec.rb
@@ -18,6 +18,11 @@ describe ReplyByEmailAddressValidator do
       expect(validator.valid_value?('foo@bar.com')).to eq(false)
     end
 
+    it "returns true if value does not contain '%{reply_key}' but 'find_related_post_with_key' is also disabled" do
+      SiteSetting.expects(:find_related_post_with_key).returns(false)
+      expect(validator.valid_value?('foo@bar.com')).to eq(true)
+    end
+
     it "returns false if value is the same as SiteSetting.notification_email" do
       SiteSetting.expects(:notification_email).returns("foo@bar.com")
       expect(validator.valid_value?('foo+%{reply_key}@bar.com')).to eq(false)