diff --git a/frontend/src/components/identities/AccountMailboxForm.vue b/frontend/src/components/identities/AccountMailboxForm.vue
index 06fd0a8db..6e69a0347 100644
--- a/frontend/src/components/identities/AccountMailboxForm.vue
+++ b/frontend/src/components/identities/AccountMailboxForm.vue
@@ -24,15 +24,21 @@
       />
   </validation-provider>
   <label class="m-label">{{ $gettext('Daily message sending limit') }}</label>
-  <v-text-field
-    v-model="form.mailbox.message_limit"
-    :placeholder="'Leave empty for no limit' | translate"
-    :hint="'Number of messages this mailbox can send per day. Leave empty for no limit.' | translate"
-    persistent-hint
-    outlined
-    dense
-    @input="update"
-    />
+  <validation-provider
+    v-slot="{ errors }"
+    rules="numeric"
+    >
+    <v-text-field
+      v-model="form.mailbox.message_limit"
+      :placeholder="'Leave empty for no limit' | translate"
+      :hint="'Number of messages this mailbox can send per day. Leave empty for no limit.' | translate"
+      persistent-hint
+      outlined
+      dense
+      :error-messages="errors"
+      @input="update"
+      />
+  </validation-provider>
 </validation-observer>
 </template>
 
@@ -94,6 +100,9 @@ export default {
         if (this.form.role === 'SimpleUsers') {
           this.$set(this.form.mailbox, 'full_address', this.form.username)
         }
+        if (newValue.mailbox.message_limit === '') {
+          this.form.mailbox.message_limit = null
+        }
       },
       deep: true
     }
diff --git a/modoboa/admin/api/v2/tests.py b/modoboa/admin/api/v2/tests.py
index fcea7e95b..819d68378 100644
--- a/modoboa/admin/api/v2/tests.py
+++ b/modoboa/admin/api/v2/tests.py
@@ -258,6 +258,48 @@ class AccountViewSetTestCase(ModoAPITestCase):
         resp = self.client.patch(url, data, format="json")
         self.assertEqual(resp.status_code, 200)
 
+    def test_message_limit_update(self):
+        account = core_models.User.objects.get(username="user@test.com")
+        url = reverse("v2:account-detail", args=[account.pk])
+        data = {
+            "username": "user@test.com",
+            "role": "SimpleUsers",
+            "password": "Toto12345",
+            "mailbox": {
+                "message_limit": 10
+            }
+        }
+        resp = self.client.patch(url, data, format="json")
+        self.assertEqual(resp.status_code, 200)
+        account.refresh_from_db()
+        self.assertEqual(account.mailbox.message_limit, 10)
+
+        data = {
+            "username": "user@test.com",
+            "role": "SimpleUsers",
+            "password": "Toto12345",
+            "mailbox": {
+                "quota": 10
+            }
+        }
+        resp = self.client.patch(url, data, format="json")
+        self.assertEqual(resp.status_code, 200)
+        account.refresh_from_db()
+        self.assertEqual(account.mailbox.message_limit, 10)
+
+        data = {
+            "username": "user@test.com",
+            "role": "SimpleUsers",
+            "password": "Toto12345",
+            "mailbox": {
+                "message_limit": None
+            }
+        }
+        resp = self.client.patch(url, data, format="json")
+        self.assertEqual(resp.status_code, 200)
+        account.refresh_from_db()
+        self.assertEqual(account.mailbox.message_limit, None)
+
     def test_update_admin(self):
         account = core_models.User.objects.get(username="admin")
         url = reverse("v2:account-detail", args=[account.pk])
diff --git a/modoboa/admin/models/mailbox.py b/modoboa/admin/models/mailbox.py
index fd2612ab1..25075b03e 100644
--- a/modoboa/admin/models/mailbox.py
+++ b/modoboa/admin/models/mailbox.py
@@ -322,7 +322,8 @@ class Mailbox(mixins.MessageLimitMixin, AdminObject):
                 not user.userobjectlimit_set.get(name="quota").max_value
             )
             self.set_quota(values["quota"], override_rules)
-        self.message_limit = values.get("message_limit")
+        if "message_limit" in values:
+            self.message_limit = values["message_limit"]
         if newaddress:
             self.rename(local_part, domain)
         self.save()