This is a maintenance and feature release, adding support for the official release of PHP 8.3, methods for removing and replacing custom headers, XCLIENT support, and links to a new way of implementing XOAUTH2 authentication.
The only change likely to have any impact on existing code is that PHPMailer previously attempted to use opportunistic STARTTLS encryption when connecting to `localhost`, which was unlikely to work. The workaround required setting `SMTPAutoTLS = false`, but that's no longer required. You may still need to use this setting when connecting to literal IPs.
References:
* [https://github.com/PHPMailer/PHPMailer/releases/tag/v6.9.1 PHPMailer 6.9.1 release notes]
* [https://github.com/PHPMailer/PHPMailer/compare/v6.8.1...v6.9.1 Full list of changes in PHPMailer 6.9.1]
Follow-up to [50628], [50799], [51169], [51634], [51635], [52252], [52749], [52811], [53500], [53535], [53917], [54427], [54937], [55557], [56484].
Props jrf, Synchro.
Fixes#59966.
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git-svn-id: http://core.svn.wordpress.org/trunk@56648 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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