diff --git a/lib/redmine/twofa/base.rb b/lib/redmine/twofa/base.rb
index e959aa930..4c9c140f9 100644
--- a/lib/redmine/twofa/base.rb
+++ b/lib/redmine/twofa/base.rb
@@ -53,7 +53,7 @@ module Redmine
       end
 
       def deliver_twofa_paired
-        Mailer.security_notification(
+        ::Mailer.security_notification(
           @user,
           User.current,
           {
@@ -82,7 +82,7 @@ module Redmine
       end
 
       def deliver_twofa_unpaired
-        Mailer.security_notification(
+        ::Mailer.security_notification(
           @user,
           User.current,
           {
@@ -114,7 +114,7 @@ module Redmine
         Token.where(user_id: @user.id).find_token('twofa_backup_code', code).try(:delete)
         # make sure the user using the backup code is the same it's been issued to
         return false unless @user.present? && @user == user_from_code
-        Mailer.security_notification(
+        ::Mailer.security_notification(
           @user,
           User.current,
           {
@@ -135,7 +135,7 @@ module Redmine
           token.update_columns value: Redmine::Utils.random_hex(6)
           tokens << token
         end
-        Mailer.security_notification(
+        ::Mailer.security_notification(
           @user,
           User.current,
           {