diff --git a/mail_outbound_static/tests/test_ir_mail_server.py b/mail_outbound_static/tests/test_ir_mail_server.py
index bbd90c1668..a9c35d1ef3 100644
--- a/mail_outbound_static/tests/test_ir_mail_server.py
+++ b/mail_outbound_static/tests/test_ir_mail_server.py
@@ -191,7 +191,7 @@ def test_02_from_outgoing_server_domaintwo(self):
     def test_03_from_outgoing_server_another(self):
         self._init_mail_server_domain_whilelist_based()
         domain = "example.com"
-        email_from = "Mitchell Admin <admin@%s>" % domain
+        email_from = f"Mitchell Admin <admin@{domain}>"
         expected_mail_server = self.mail_server_domainone
 
         self.message.replace_header("From", email_from)