Set content type to HTML for lost password email only

The filter to manage content type is ‘wp_mail_content_type’. You could add this filter to set the next email to the desired type from another callback used exclusively by the lost password process, ‘retrieve_password_message’ for example. You don’t need to actually change the message, just return the passed message unchanged. You are using this filter like an action hook because it only fires for password resets and it fires before the ‘wp_mail_content_type’ filter.

I don’t think I follow. I’m using wp_mail_content_type to set it like so:

function xxx_wp_email_content_type() {
	return 'text/html';
}
add_filter( 'wp_mail_content_type', 'xxx_wp_email_content_type' );

And I have another function that changes the content of the email:

function xxx_wp_retrieve_password_message( $content, $key ) {
	ob_start();
	
	$email_subject = xxx_wp_retrieve_password_title();
	
	include( 'templates/email_header.php' );
	include( 'templates/lost_password_email.php' );
	include( 'templates/email_footer.php' );
	
	$message = ob_get_contents();
	ob_end_clean();

	return $message;
}
add_filter( 'retrieve_password_message', 'xxx_wp_retrieve_password_message', 10, 2 );

Do you mean I should call xxx_wp_email_content_type() inside xxx_wp_retrieve_password_message()?