diff --git a/core/modules/system/config/schema/system.schema.yml b/core/modules/system/config/schema/system.schema.yml
index 5148f5347fd48d52de3b2c3fd42df0dff9dec67c..40336b7934a5ebf009a875a946b1de9a70896917 100644
--- a/core/modules/system/config/schema/system.schema.yml
+++ b/core/modules/system/config/schema/system.schema.yml
@@ -366,6 +366,8 @@ system.image.gd:
 system.mail:
   type: config_object
   label: 'Mail system'
+  constraints:
+    FullyValidatable: ~
   mapping:
     interface:
       type: sequence
@@ -397,10 +399,16 @@ system.mail:
           type: string
           nullable: true
           label: 'User'
+          constraints:
+            NotBlank:
+              allowNull: true
         password:
           type: string
           nullable: true
           label: 'Password'
+          constraints:
+            NotBlank:
+              allowNull: true
         port:
           type: integer
           nullable: true