diff --git a/Lib/smtplib.py b/Lib/smtplib.py
index 13c56e196f8..fbef96e691f 100644
--- a/Lib/smtplib.py
+++ b/Lib/smtplib.py
@@ -912,6 +912,7 @@ if __name__ == '__main__':
 
     def prompt(prompt):
         sys.stdout.write(prompt + ": ")
+        sys.stdout.flush()
         return sys.stdin.readline().strip()
 
     fromaddr = prompt("From")
diff --git a/Misc/NEWS b/Misc/NEWS
index fbba9b1626f..d610ff45f21 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -47,6 +47,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #12448: smtplib now flushes stdout while running ``python -m smtplib``
+  in order to display the prompt correctly.
+
 - Issue #13194: zlib.compressobj().copy() and zlib.decompressobj().copy() are
   now available on Windows.