diff --git a/config/locales/server.en.yml b/config/locales/server.en.yml
index faa880d169f..8118d7b3409 100644
--- a/config/locales/server.en.yml
+++ b/config/locales/server.en.yml
@@ -915,6 +915,7 @@ en:
 
     errors:
       invalid_email: "Invalid email address."
+      invalid_username: "There's no user with that username."
 
   notification_types:
     mentioned: "%{display_username} mentioned you in %{link}"
diff --git a/config/site_settings.yml b/config/site_settings.yml
index e1bb6d2de6f..addcc309d7d 100644
--- a/config/site_settings.yml
+++ b/config/site_settings.yml
@@ -9,7 +9,9 @@ required:
   notification_email:
     default: 'info@discourse.org'
     validator: 'EmailSettingValidator'
-  site_contact_username: ''
+  site_contact_username:
+    default: ''
+    validator: 'UsernameSettingValidator'
   logo_url:
     client: true
     default: '/images/d-logo-sketch.png'
@@ -316,7 +318,9 @@ email:
   pop3s_polling_enabled: false
   pop3s_polling_host: ''
   pop3s_polling_port: 995
-  pop3s_polling_username: ''
+  pop3s_polling_username:
+    default: ''
+    validator: 'UsernameSettingValidator'
   pop3s_polling_password: ''
   email_in:
     default: false
@@ -484,7 +488,9 @@ embedding:
   embeddable_host: ''
   feed_polling_enabled: false
   feed_polling_url: ''
-  embed_by_username: ''
+  embed_by_username:
+    default: ''
+    validator: 'UsernameSettingValidator'
   embed_category: ''
   embed_post_limit: 100
   embed_truncate: false
diff --git a/lib/validators/email_setting_validator.rb b/lib/validators/email_setting_validator.rb
index fae1c78a589..f499fb22597 100644
--- a/lib/validators/email_setting_validator.rb
+++ b/lib/validators/email_setting_validator.rb
@@ -1,6 +1,6 @@
 class EmailSettingValidator
   def self.valid_value?(val)
-    val == '' || EmailValidator.email_regex =~ val
+    !val.present? || !!(EmailValidator.email_regex =~ val)
   end
 
   def self.error_message(val)
diff --git a/lib/validators/username_setting_validator.rb b/lib/validators/username_setting_validator.rb
new file mode 100644
index 00000000000..ce05cae5c27
--- /dev/null
+++ b/lib/validators/username_setting_validator.rb
@@ -0,0 +1,9 @@
+class UsernameSettingValidator
+  def self.valid_value?(val)
+    !val.present? || User.where(username: val).exists?
+  end
+
+  def self.error_message(val)
+    I18n.t('site_settings.errors.invalid_username')
+  end
+end
diff --git a/spec/components/validators/email_setting_validator_spec.rb b/spec/components/validators/email_setting_validator_spec.rb
new file mode 100644
index 00000000000..c7ba6e5dc7e
--- /dev/null
+++ b/spec/components/validators/email_setting_validator_spec.rb
@@ -0,0 +1,18 @@
+require 'spec_helper'
+
+describe EmailSettingValidator do
+  describe '#valid_value?' do
+    it "returns true for blank values" do
+      described_class.valid_value?('').should == true
+      described_class.valid_value?(nil).should == true
+    end
+
+    it "returns true if value is a valid email address" do
+      described_class.valid_value?('vader@example.com').should == true
+    end
+
+    it "returns false if value is not a valid email address" do
+      described_class.valid_value?('my house').should == false
+    end
+  end
+end
diff --git a/spec/components/validators/username_setting_validator_spec.rb b/spec/components/validators/username_setting_validator_spec.rb
new file mode 100644
index 00000000000..52da3c9a571
--- /dev/null
+++ b/spec/components/validators/username_setting_validator_spec.rb
@@ -0,0 +1,19 @@
+require 'spec_helper'
+
+describe UsernameSettingValidator do
+  describe '#valid_value?' do
+    it "returns true for blank values" do
+      described_class.valid_value?('').should == true
+      described_class.valid_value?(nil).should == true
+    end
+
+    it "returns true if value matches an existing user's username" do
+      Fabricate(:user, username: 'vader')
+      described_class.valid_value?('vader').should == true
+    end
+
+    it "returns false if value does not match a user's username" do
+      described_class.valid_value?('no way').should == false
+    end
+  end
+end