diff -ruN original/devel.install mod/devel.install
--- original/devel.install	2011-07-22 23:20:44.000000000 +0700
+++ mod/devel.install	2011-09-16 15:16:31.000000000 +0700
@@ -94,8 +94,18 @@
     drupal_set_message(t('Disabled query logging since devel module is disabled.'));
   }
   
-  // The SMTP server should also be restored.
-  variable_set('smtp_library', variable_get('devel_old_smtp_library', ''));
+  // The SMTP server should also be restored, but only if it was set to devel
+  // module or to default sending method.
+  $old_smtp = variable_get('devel_old_smtp_library', NULL);
+  $current_smtp = variable_get('smtp_library', NULL);
+  if (empty($current_smtp) || $current_smtp == drupal_get_filename('module', 'devel')) {
+    if (empty($old_smtp)) {
+      variable_del('smtp_library');
+    }
+    else {
+      variable_set('smtp_library', $old_smtp);
+    }
+  }
   variable_del('devel_old_smtp_library');
   
   // Same for storing queries