SMTP authentication is optional (#2765)

Co-authored-by: Laurent Magnien <laurent.magnien@adsn.fr>

api/extensions/ext_mail.py

                 from libs.smtp import SMTPClient
                 if not app.config.get('SMTP_SERVER') or not app.config.get('SMTP_PORT'):
                     raise ValueError('SMTP_SERVER and SMTP_PORT are required for smtp mail type')
-                if not app.config.get('SMTP_USERNAME') or not app.config.get('SMTP_PASSWORD'):
-                    raise ValueError('SMTP_USERNAME and SMTP_PASSWORD are required for smtp mail type')
                 self._client = SMTPClient(
                     server=app.config.get('SMTP_SERVER'),
                     port=app.config.get('SMTP_PORT'),

api/libs/smtp.py

         smtp = smtplib.SMTP(self.server, self.port)
         if self._use_tls:
             smtp.starttls()
-        smtp.login(self.username, self.password)
+        if (self.username):
+            smtp.login(self.username, self.password)
         msg = MIMEMultipart()
         msg['Subject'] = mail['subject']
         msg['From'] = self._from