Can't Connect to SMTP Host - Office 365

ℹ️ Support
7

@pauldow From what I can see the error message is issued in only one place in phpmailer. You can try adding some extra debug statements to see exactly where it is failing
in file admin/PHPMailer/class.smtp.php replace the startTLS() function

public function startTLS()
{
    if (!$this->sendCommand('STARTTLS', 'STARTTLS', 220)) {
        $this->edebug('send command failed', self::DEBUG_CONNECTION);
        return false;
    }

    //Allow the best TLS version(s) we can
    $crypto_method = STREAM_CRYPTO_METHOD_TLS_CLIENT;

    //PHP 5.6.7 dropped inclusion of TLS 1.1 and 1.2 in STREAM_CRYPTO_METHOD_TLS_CLIENT
    //so add them back in manually if we can
    if (defined('STREAM_CRYPTO_METHOD_TLSv1_2_CLIENT')) {
        $crypto_method |= STREAM_CRYPTO_METHOD_TLSv1_2_CLIENT;
        $crypto_method |= STREAM_CRYPTO_METHOD_TLSv1_1_CLIENT;
    }

    // Begin encrypted connection
    if (!stream_socket_enable_crypto(
        $this->smtp_conn,
        true,
        $crypto_method
    )) {
        $this->edebug('streamsocket failed', self::DEBUG_CONNECTION);
        return false;
    }

    $this->edebug('start TLS successful', self::DEBUG_CONNECTION);
    return true;
}

Set the smtp debug level to be 3 then send a test email. With your settings I get “start TLS successful” then it fails on authentication