phpmailer_init 每次都会开火wp_mail() 调用-但是，您可以有条件地如下挂接/取消挂接：

function wpse_224496_phpmailer_init( $phpmailer ) {
    // SMTP setup

    // Always remove self at the end
    remove_action( \'phpmailer_init\', __function__ );
}

function wpse_224496_wp_mail( $mail ) {
    // Example: only SMTP for emails addressed to [email protected]
    if ( $mail[\'to\'] === \'[email protected]\' )
        add_action( \'phpmailer_init\', \'wpse_224496_phpmailer_init\' );

    // Example: only SMTP for subject "Foo"
    if ( $mail[\'subject\'] === \'Foo\' )
        add_action( \'phpmailer_init\', \'wpse_224496_phpmailer_init\' );

    // Other properties
    $mail[\'message\'];
    $mail[\'headers\']; // Could be string or array
    $mail[\'attachments\']; // Could be string or array

    return $mail;
}

add_filter( \'wp_mail\', \'wpse_224496_wp_mail\' );