[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):

 Hello,

 I understand that this ticket primarily addresses a PHP error related to
 setting the Envelope-From.

 However, I would like to comment on the broader decision to always define
 an Envelope-From address, which is related but separate from the PHP error
 itself.

 The concern here is not whether an Envelope-From should exist, but that
 RFC standards require the Envelope-From to be a valid mailbox. In many
 real-world hosting environments, wordpress at domain.tld does not exist and
 cannot be sure that it exists — particularly when email is hosted
 externally.

 The rationale given for defining an Envelope-From by default is to help
 hosts that do not configure it correctly.

 However, defining it with an invalid mailbox has the following
 consequences:

 - It reduces deliverability for WordPress sites on hosts that already
 configure return paths correctly and prevents from receiving delivery
 errors
 - It negatively impacts IP reputation on hosts that do not enforce RFC
 compliance.
 - It blocks email entirely on hosts that do enforce RFC compliance and
 reject invalid Envelope-From addresses.

 While I agree that leaving the Envelope-From undefined is not ideal for
 deliverability, it is still preferable to defining one that is invalid, as
 the latter actively breaks previously working and RFC-compliant setups.

 Given this, I would kindly ask the team to reconsider defining the
 Envelope-From by default until a more complete solution is available — or
 at least make it opt-in — ideally with:

 - a configurable setting for the Envelope-From address, and/or
 - detection and admin warnings when the configured Envelope-From is likely
 invalid.

 This would avoid breaking existing, correctly configured sites until
 WordPress can improve email handling by allowing users to define the
 "Envelope From" or the E-mail sender from WordPress's interface.

-- 
Ticket URL: <https://core.trac.wordpress.org/ticket/64368#comment:42>
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