Deprecate "Post via email" feature

[ayeshrajans]

While working on a ticket to upgrade WordPress's email library (PHPMailer) to the latest version, the author of the PHPMailer library commented about the POP3 library.

WordPress uses this POP3 class from within ./wp-mail.php file to connect to a POP3 server, fetch emails, and post them on the WordPress site.

Frankly I feel a little unwell at the thought that some might still be using the POP3 library! Nobody should be using POP-before-SMTP for auth any more - the last time I used it was about 30 years ago. Indeed it doesn't have any test coverage - it was originally a third party library that was donated to the project, and it's a difficult thing to test as it requires coordination across protocols. Do you have any visibility on whether it's being used at all?

Emails are hard by nature, and converting an email to a WordPress is quite difficult with the complexities involved in converting the 7-bit encoding that are common in emails to the modern UTF-8 characters, connection security, contect security, etc. Furthermore, with Gutenberg and numerous media improvements, the "Post via email" feature is lacking way behind.

I propose to deprecate the "Post via email" feature (version 5.4 would be amazing!), and eventually remove it.

• wp-mail.php file is not tested. While working on PHPMailer 6 upgrade patch, only one test failed.

• Commentary by the PHPMailer library author about the POP3 class and how less active the development of it. The history of this file reveals only code styling and modernization changes (such as namespaces, doc comments, etc) were made. No bug fixes.

• Possible security vulnerabilities ​such as this one

• Possibility of moving this functionality to a contributed plugin, so even the (likely small percentage) users of this feature have an upgrade path.

[ayeshrajans]

Related: #41750

[SergeyBiryukov]

Thanks for the ticket, we're already tracking this enhancement in #22942.