[MM-54594] Adding new global relay type (#24672)

* adding new global relay type, custom

server/i18n/en.json

@@ -8848,7 +8848,11 @@
   },
   {
     "id": "model.config.is_valid.message_export.global_relay.customer_type.app_error",
-    "translation": "Message export GlobalRelaySettings.CustomerType must be set to one of either 'A9' or 'A10'."
+    "translation": "Message export GlobalRelaySettings.CustomerType must be set to one of either 'A9', 'A10' or 'CUSTOM."
+  },
+  {
+    "id": "model.config.is_valid.message_export.global_relay.customer_type_custom.app_error",
+    "translation": "If GlobalRelaySettings.CustomerType is 'CUSTOM', then GlobalRelaySettings.CustomSMTPServerName and GlobalRelaySettings.CustomSMTPPort must be set."
   },
   {
     "id": "model.config.is_valid.message_export.global_relay.email_address.app_error",