[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 amanandhishoe):

 Replying to [comment:64 SirLouen]:

 It seems like you are conflating the **Reply-To header** with the **SMTP
 envelope Return-Path**. The issue is that WordPress may attempt to
 influence the **envelope Return-Path**, but that address must be
 deliverable, since it is where mail servers send bounce notifications.

 They are different layers and serve different purposes:
         •       **Reply-To**
         •       An RFC 5322 message header
         •       Controls where a human’s email client sends replies
         •       Has no role in bounces, SPF, or IP reputation
         •       **Envelope Return-Path**
         •       Derived from the SMTP envelope MAIL FROM
         •       Used exclusively for bounces and delivery failures
         •       Evaluated by MTAs for SPF alignment, bounce handling, and
 reputation
 The comment you quoted repeatedly slides between these two, as if they
 were interchangeable. They are not

 Modern email systems do not care if the **Reply-To address** is invalid.
 They do care if the **Return-Path address** is not deliverable. They also
 care a great deal whether the domain exists, whether SPF aligns, and
 whether **bounces can be accepted somewhere**.

 The claim that this is a “2000s issue” is outdated. Today, envelope
 handling affects SPF alignment, DMARC evaluation, backscatter handling,
 and reputation scoring at major providers. Whether **SpamAssassin** is
 involved is irrelevant; it is no longer the primary gatekeeper. Reputation
 systems at Gmail, Outlook, Yahoo, and others operate independently.

 It is not just a matter of “there are plenty of easy fixes, like … catch-
 all.” Those are host-level decisions, not application-level ones.
 WordPress does not know the MTA, the rewriting rules, the SPF topology, or
 whether a catch-all exists.

 You are correct when you say, “What massively destroys the IP reputation
 is to mass send emails to nonexistent inboxes.” But it is unrelated to the
 envelope discussion and does not justify WordPress intervening at the SMTP
 layer.

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Ticket URL: <https://core.trac.wordpress.org/ticket/64368#comment:65>
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