diff --git a/lib/validators/reply_by_email_address_validator.rb b/lib/validators/reply_by_email_address_validator.rb
index ff668bf8582..a9110e0f850 100644
--- a/lib/validators/reply_by_email_address_validator.rb
+++ b/lib/validators/reply_by_email_address_validator.rb
@@ -4,14 +4,19 @@ class ReplyByEmailAddressValidator
   end
 
   def valid_value?(val)
+    val&.strip!
+
     return true  if val.blank?
-    return false if val["@"].nil?
+    return false if !val.include?("@")
+
+    value = val.dup
 
     if SiteSetting.find_related_post_with_key
-      !!val["%{reply_key}"] && val.sub(/\+?%{reply_key}/, "") != SiteSetting.notification_email
-    else
-      val != SiteSetting.notification_email
+      return false if !value.include?("%{reply_key}")
+      value.sub!(/\+?%{reply_key}/, "")
     end
+
+    value != SiteSetting.notification_email && !value.include?(" ")
   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 cf7a889a4a7..c64a4a62a5e 100644
--- a/spec/components/validators/reply_by_email_address_validator_spec.rb
+++ b/spec/components/validators/reply_by_email_address_validator_spec.rb
@@ -12,6 +12,7 @@ describe ReplyByEmailAddressValidator do
 
     it "returns false if value is not an email address" do
       expect(validator.valid_value?('WAT%{reply_key}.com')).to eq(false)
+      expect(validator.valid_value?('word +%{reply_key}@example.com')).to eq(false)
     end
 
     it "returns false if value does not contain '%{reply_key}'" do