This is a simple email sender application built using Tkinter, a GUI toolkit for Python, that allows you to send emails with HTML content. The application provides a user-friendly interface where you can enter the email sender's credentials, recipient, subject, and body of the email. It also supports saving the email credentials and logging out.

To run this application, you need to have the following prerequisites:

• Python 3.x installed on your system
• tkinter library installed (usually included in Python standard library)
• beautifulsoup4 library installed (can be installed using pip install beautifulsoup4)
• cssutils library installed (can be installed using pip install cssutils)
• requests library installed (can be installed using pip install requests)
• smtplib library installed (usually included in Python standard library)

1. Clone the repository or download the source code files.

2. Open a terminal or command prompt and navigate to the directory where the files are located.

3. Run the following command to start the application:

   python email_sender_app.py

4. The Email Sender App window will open.

5. Enter the email address and password of the email sender in the respective fields.

6. Enter the recipient's email address in the "Recipient" field.

7. Enter the subject of the email in the "Subject" field.

8. Enter the body of the email in the "Body" text area. You can use HTML tags and inline CSS to format the content.

9. Click the "Save Email and password" button to save the email credentials (optional).

10. Click the "Send Email" button to send the email. The HTML content will be processed to convert any external CSS into inline styles before sending.

11. To log out, click the "Logout" button. This will remove the saved email credentials (if any) and clear the input fields.

Note: If there is an existing email_credentials.txt file in the same directory as the application, the email and password fields will be automatically filled with the saved credentials when the application starts.

• The application uses the tkinter library to create the graphical user interface.

• The beautifulsoup4 library is used to parse the HTML content and convert external CSS into inline styles.

• The cssutils library is used to parse and manipulate CSS styles.

• The requests library is used to fetch external CSS files.

• The smtplib library is used to connect to the SMTP server and send the email.

• The email is sent using the Gmail SMTP server. If you want to use a different email provider, you need to modify the SMTP server details in the emailsender() function.

• The email credentials are saved in a text file named email_credentials.txt in the same directory as the application. The file format is as follows:

  Email: [email_address]
  Password: [password]
  

• The save_email() function is responsible for saving the email credentials to the file.

• The logout() function deletes the email_credentials.txt file and clears the input fields.

• The style_to_inline() function converts any external CSS styles in the HTML content to inline styles before sending the email.

• The application provides basic error handling and validation. However, it is recommended to perform additional testing and validation as per your specific requirements.

• The application's GUI elements are styled with default Tkinter styling. You can customize the styling further if desired.

Feel free to modify and enhance the application as needed to suit your requirements.