gh-136134: Add code comment explaining CRAM-MD5 fallback and NEWS entry

Author: nogueira-raphael

Lib/smtplib.py

@@ -744,6 +744,9 @@ def login(self, user, password, *, initial_response_ok=True):
             except SMTPAuthenticationError as e:
                 last_exception = e
             except ValueError as e:
+                # Some environments (e.g., FIPS) disable certain hashing algorithms like MD5,
+                # which are required by CRAM-MD5. This raises a ValueError when trying to use HMAC.
+                # If this happens, we catch the exception and continue trying the next auth method.
                 last_exception = e
                 if 'unsupported' in str(e).lower():
                     continue

Misc/NEWS.d/next/Library/2025-07-01-21-37-17.gh-issue-136134.NhFpu3.rst

@@ -0,0 +1,2 @@
+Make smtplib fallback to next auth method if CRAM-MD5 fails due to
+unsupported hash (e.g. FIPS). Contributed by Raphael Rodrigues.