gh-70039: smtplib: store the server name in ._host in .connect()

[nmartensen]

Original patch by gigaplastik, extended with a few more tests.

Addresses:

• Issue: smtplib's SMTP.connect() should store the server name in ._host for .starttls() #70039

• Issue: When using an SMTP SSL connection,, get ValueError. #80275

[ghost]

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[bedevere-app]

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[nmartensen]

The PR adds 6 tests in addition to the functional change. Most of them fail without the change:

• In Lib/test/test_smtplib.py:DebuggingServerTests
  • test_host_port_host [fails] *
  • test_explicit_connect [fails]
  • text_explicit_connect_host_port [fails]
• In Lib/test/test_smtpnet.py:SmtpTest
  • test_connect_host_port_starttls
  • test_explicit_connect_starttls [fails]
  • test_explicit_connect_host_port_starttls [fails]

The one that does not fail describes a situation that could sometimes also fail in practice before the change, see e.g. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=926900. The test marked with '*' above is meant to ensure this failure case won't be accidentally reintroduced in potential future changes.

[AliceGrey]

Any chance of getting this reviewed and merged? This bug has been open since 2019 and is still causing issues. This PR is excellent. It fixes the issue and adds several tests.

[jahs]

Hi all, I'd like to echo the above as this is still an issue (going back to 2015). Caused an issue this week (December 2025) due to AWS SES eu-west-2 applying strict parsing in TLS of the self._host string (which they rolled back, in part due to this client bug). Specifically, the SMTP("host:port") constructor was used and it failed to connect, as the connect() parsing currently does not update self._host, which is then used in starttls().

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[nmartensen]

I have made the requested changes; please review again.

[bedevere-app]

Thanks for making the requested changes!

@bitdancer: please review the changes made to this pull request.

[nmartensen]

There seem to be failing tests with this PR now. I have looked into them but fail to see what is needed to fix them. Please let me know if you know more.

[bitdancer]

I think all you need to do is update to the tip of main. Let's try that, at least.