This reminds me of #58650. We can reopen it, but I would still like to understand what we are missing here (see the discussion in the linked PR).

@xabbuh After looking into it, it seems the issue is very specific and only happens on Windows. I couldn't recreate it with Mailpit, so I set an environment variable to use Mailtrap with bad credentials.

Minimal reproducer: https://github.com/skmedix/symfony-issue-59351/blob/master/.github/workflows/smtp-runner.yaml

Failed run: https://github.com/skmedix/symfony-issue-59351/actions/runs/12597218959/job/35109776421

Puzzle solved. Debugged this even deeper, as it kept bugging me! So I persisted to find out. It took me a couple of hours, but I discovered that the error_get_last() is NULL on both systems, but this warning does not appear on Linux because the line 92 is never reached there, whereas it is on Windows. The problem comes with the stream_get_meta_data function (here) and it's return value, while on Linux the last command returns [eof => true], but on Windows it's [eof => false]. That's why on Linux the exception is thrown earlier on $metas['eof'], while on Windows the code goes further and throws a warning because error_get_last() is null.

Last stream_get_meta_data var_dump diff:

-   ["eof"]=>
-   bool(true)
+   ["eof"]=>
+   bool(false)

I tried changing $metas['eof'] to use \feof($this->out) and this solution works both on windows and linux, I guess now I can make a PR :)

Edit: Here's a run with PR #59404 applied as a patch. Both steps (test and retest) show the results before and after applying the fix:
https://github.com/skmedix/symfony-issue-59351/actions/runs/12659627195/job/35279113129#annotation:6:19
https://github.com/skmedix/symfony-issue-59351/actions/runs/12659627195/job/35279113129#annotation:9:22