Error while launching example code

```
Traceback (most recent call last):
File "C:\WorkFolder\secure-mail-server\server\server.py", line 1, in
from secure_smtpd import SMTPServer, FakeCredentialValidator
File "C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\site-packages\secure_smtpd\__init__.py", line 3, in
from .smtp_server import SMTPServer
File "C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\site-packages\secure_smtpd\smtp_server.py", line 4, in
from .smtp_channel import SMTPChannel
File "C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\site-packages\secure_smtpd\smtp_channel.py", line 5, in
from smtpd import NEWLINE, EMPTYSTRING
ImportError: cannot import name 'EMPTYSTRING' from 'smtpd' (C:\Users\Admin\AppData\Local\Programs\Python\Python310\lib\smtpd.py)
```

while launching

```
from secure_smtpd import SMTPServer, FakeCredentialValidator
SMTPServer(
self,
('0.0.0.0', 465),
None,
require_authentication=True,
ssl=True,
certfile='cert.crt',
keyfile='key.key',
credential_validator=FakeCredentialValidator(),
)
asyncore.loop()
```

Reproduce the launch failure from the example code in server/server.py and inspect secure_smtpd/smtp_channel.py, especially its import from Python's smtpd module. Confirm compatibility with Python 3.10, then verify that the example starts without the reported ImportError.

Written by the indexing model from the issue text.