[wp-trac] [WordPress Trac] #64368: `Could not instantiate mail function` errors sending mail in 6.9

[WordPress Trac]

#64368: `Could not instantiate mail function` errors sending mail in 6.9
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 Reporter:  desrosj       |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  6.9.1
Component:  Mail          |    Version:  6.9
 Severity:  normal        |   Keywords:  needs-testing
  Focuses:                |
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 There have been some reports of `wp_mail()` failing to send emails after
 upgrading to 6.9. This was
 [https://www.reddit.com/r/Wordpress/comments/1pdu68r/wordpress_doesnt_send_emails_after_69_update/
 first spotted on the `r/Wordpress` subreddit], but there have been a few
 other reports since (#64367 here on Trac).

 The culprit seems to be [61010] which solves #49687. Reverting that
 specific change (or downgrading to 6.8.3) has fixed the problem for every
 tester so far.

 Because #49687 is a closed ticket on a completed milestone, this is being
 created to track this bug and eventually reference in any commits that are
 deemed necessary (if any at all).

 The discussion about the problem begins at in the previous ticket with
 ticket:49687#comment:44. While it's worth reading through that
 conversation, here are some notes from the discussion so far:
 - The issue mentioned as a possible problem in ticket:49687#23 seems like
 it could be causing these issues.
 - Including `-f` when setting `sendmail_from` is bad practice, but is a
 widespread misunderstanding due to upstream documentation in PHP. An
 [https://github.com/php/php-src/issues/20648 upstream ticket in] `php-src`
 has been opened to address this.
 - While `-f` is not supposed to be passed to
 `php_admin_value[sendmail_path]`, using `php_admin_value[sendmail_from]`
 does not seem to work as expected for some in Linux environments using
 Postfix with Sendmail compatibility interface.

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Ticket URL: <https://core.trac.wordpress.org/ticket/64368>
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