bug(smtp): RCPT TO rejection kills entire send instead of skipping invalid recipient

[usernane]

Bug Description

In SMTPServer::sendCommand(), any response code >= 400 from the previous command causes the next sendCommand() call to throw immediately:

if ($this->lastResponseCode >= 400) {
    throw new SMTPException(...);
}

This is incorrect for RCPT TO commands. In SMTP, a 5xx response to RCPT TO is a per-recipient rejection — not a session failure. The client should continue sending RCPT TO for remaining recipients and only fail if all are rejected.

Steps to Reproduce

1. Create an email with multiple recipients: valid@example.com, invalid@example.com, also-valid@example.com
2. The SMTP server rejects invalid@example.com with 550 User not found
3. The next sendCommand() call (for also-valid@example.com) throws SMTPException without even being sent

Expected Behavior

• Rejected RCPT TO addresses should be collected/logged but not abort the session
• Remaining recipients should still be attempted
• DATA should proceed if at least one recipient was accepted
• Only throw if all recipients were rejected
• QUIT should always be sent to cleanly close the connection

Actual Behavior

• First RCPT TO rejection aborts the entire send
• Remaining valid recipients never receive the message
• QUIT is never sent, leaving the TCP connection dangling until server timeout
• No way to know which recipients were accepted vs rejected

WebFiori Version

v2.1.1

PHP Version

8.1+

Operating System

Linux

Additional Context

Suggested approach — modify receiversCommandHelper() to handle rejections gracefully:

private function receiversCommandHelper($type): int {
    $server = $this->getSMTPServer();
    $accepted = 0;

    foreach ($this->receiversArr[$type] as $address => $name) {
        $server->sendCommand('RCPT TO: <' . $address . '>');

        if ($server->getLastResponseCode() < 400) {
            $accepted++;
        }
        // Log rejection but don't throw
    }

    return $accepted;
}

Then in send(), check total accepted count and throw only if zero.

This requires sendCommand() to not throw on the next call after a 4xx/5xx — the check should be contextual rather than blanket.