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When using WP Mail SMTP to send an HTML email, the phpmailer AltBody content is not being set.
I guess probably this is happening with all providers because when there’s a Body and an AltBody, phpmailer sets the content type to multipart/alternative and not text/html.
On MailerAbstract.php, line 108, you’re only looking at text/html to determine if you’re going to set both HTML and text versions, or else you’ll only set one version.
This is easily fixed by changing:
if ( $this->phpmailer->ContentType === 'text/html' ) {
to:
if ( $this->phpmailer->ContentType === 'text/html' || $this->phpmailer->ContentType === 'multipart/alternative' ) {