fix: Use SMTP server instead of gmail

The script was using gmail as SMTP server. Or the script uses
a custom SMTP one.

session_notifier.py

@@ -1,8 +1,10 @@
 import smtplib
 import os
+import logging
 from email.message import EmailMessage
 from telegram import Bot
 
+
 class SessionNotifier:
     """
     A class to handle notifications for session bookings.
@@ -39,6 +41,9 @@ class SessionNotifier:
         Args:
             message (str): The message content to be sent in the email
         """
+        logging.debug("Sending email notification")
+        logging.debug(f"Email credentials: {self.email_credentials}")
+
         # Create an EmailMessage object
         email = EmailMessage()
         email.set_content(message)
@@ -51,9 +56,21 @@ class SessionNotifier:
         email['Subject'] = 'Session Booking Notification'
 
         # Send the email using smtplib
-        with smtplib.SMTP_SSL('smtp.gmail.com', 465) as smtp:
-            smtp.login(self.email_credentials['from'], self.email_credentials['password'])
-            smtp.send_message(email)
+        try:
+            smtp_server = os.environ.get("SMTP_SERVER")
+            if not smtp_server:
+                logging.error("SMTP server not configured in environment variables")
+                raise ValueError("SMTP server not configured")
+
+            with smtplib.SMTP_SSL(smtp_server, 465) as smtp:
+                logging.debug(f"Connecting to SMTP server: {smtp_server}")
+                smtp.login(self.email_credentials['from'], self.email_credentials['password'])
+                logging.debug("Logged in to SMTP server")
+                smtp.send_message(email)
+                logging.debug("Email sent successfully")
+        except Exception as e:
+            logging.error(f"Failed to send email: {str(e)}")
+            raise
 
     def send_telegram_notification(self, message):
         """