[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:  SirLouen
     Type:  defect (bug)              |      Status:  accepted
 Priority:  normal                    |   Milestone:  6.9.1
Component:  Mail                      |     Version:  6.9
 Severity:  normal                    |  Resolution:
 Keywords:  has-patch has-unit-tests  |     Focuses:
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Comment (by jorbin):

 Replying to [comment:80 SirLouen]:
 > With those premises, this is completely untestable. Precisely, things
 like RC here are completely useless because no one uses them anywhere near
 production (reason why no one caught this simple issue, which was
 theoretically "affecting deliverability").

 Ensuring there is enough testing is always a challange, but that doesn't
 mean something is completely untestable. It's unfortunate that people's
 website broke because of the change in [61010]. The goal of 6.9.1 is to
 fix that, and reverting this change allows that to be done in a way that I
 have the greatest confidence in.

 > The same is going to happen again here: no "reporter" is going to test
 any move towards 7.0. As Johnatan said, there was not enough critical mass
 to consider that this regression was urgent for a prompt 6.9.1 release for
 a reason is apparently impacting only a little subset of users with not
 ideally configured hosts according to hosting good practices.

 A quick 6.9.1 was not done due to the ability to provide a workaround in
 the hotfix plugin, a desire to not do multiple releases close to a time
 when many people are taking time off, and a desire to understand the issue
 more completely. See
 https://make.wordpress.org/core/2025/12/12/wordpress-6-9-hotfixes/ for
 more information on this decision.

 It's unknown how many users are effected since users may not know they are
 not getting emails. It's unknown how much of this is directly due to the
 sendmail_path collision and how many are due to setting a different domain
 for outgoing mail. These unknowns lead me to believing that shipping code
 that is known to work in production environments as the best solution.

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Ticket URL: <https://core.trac.wordpress.org/ticket/64368#comment:83>
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