[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     |     Focuses:
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Comment (by Monarobase):

 We agree that DMARC alignment is a very important deliverability signal,
 and this was indeed a limitation of the pre-6.9 behavior.

 **DMARC failure**
 - Message is usually still delivered
 - Worst case: lands in spam or quarantine
 - No protocol violation

 **Invalid return-path**
 - Message can be rejected outright
 - Bounce handling is impossible
 - Violates RFC expectations for a bounceable sender

 A DMARC failure degrades deliverability of individual e-mails while an
 invalid return-paths break mail flow and reduce IP reputation when this
 happens. IP reptation affects all users and not only WordPress users.

 We are not just thinking from a WordPress perspective, we are thinking for
 all of our customers. Should non WordPress users be impacted because the
 people who decide about allowing a new field in the admin interface don't
 consider fixing e-mail deliverability issues is important to them ?

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Ticket URL: <https://core.trac.wordpress.org/ticket/64368#comment:63>
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