WP-r61201: Mail: Improve multipart message handling in wp_mail()
#2193
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This improves how `wp_mail()` handles `Content-Type` headers for multipart messages, preventing cases where the header could be duplicated. Developed in WordPress/wordpress-develop#9500 WP:Props SirLouen, gitlost, rmccue, westi, MattyRob, bgermann, nacin, SergeyBiryukov, dd32, MikeHansenMe, Kleor, kitchin, JeffMatson, abcd95, westonruter, christinecooper, JohnVieth, dawidadach, imokweb, ayeshrajans, lakshyajeet, tusharbharti, sajjad67. Fixes https://core.trac.wordpress.org/ticket/15448. --- Merges https://core.trac.wordpress.org/changeset/61201 / WordPress/wordpress-develop@024bbfcc4c to ClassicPress.
Changed the type hint from WP_PHPMailer to MockPHPMailer in the $set_alt_body callback within the wpMail test. This ensures the callback matches the actual mock object used in the test environment.
xxsimoxx
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Nov 13, 2025
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Before:
Content-Type: =?us-ascii?Q?multipart/alternative;_boundary=3D"----=3D=5FPa?=
=?us-ascii?Q?rt=5F18243133=5F1346573420.1408991447668"?=
MIME-Version: 1.0
Content-Type: multipart/alternative; charset=
After:
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="----=_Part_18243133_1346573420.1408991447668"; charset=
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Description
This improves how
wp_mail()handlesContent-Typeheaders for multipart messages, preventing cases where the header could be duplicated.Motivation and context
Backport and fixes potential header duplication.
How has this been tested?
Unit test included and fixed for ClassicPress.
Screenshots
N/A
Types of changes