Sending simple emails in Flask - Step-by-Step Execution

Concept Flow - Sending simple emails

Start Flask app

↓

User triggers email send

↓

Create email message object

↓

Configure SMTP server connection

↓

Send email through SMTP

↓

Close SMTP connection

↓

Return success/failure response

This flow shows how a Flask app sends an email: user triggers sending, app creates message, connects to SMTP server, sends email, then closes connection.

Execution Sample

Flask

from flask import Flask
from flask_mail import Mail, Message

app = Flask(__name__)
app.config.update(
  MAIL_SERVER='smtp.example.com',
  MAIL_PORT=587,
  MAIL_USE_TLS=True,
  MAIL_USERNAME='[e-mail]',
  MAIL_PASSWORD='password'
)
mail = Mail(app)

@app.route('/send')
def send_email():
  msg = Message('Hello', sender='[e-mail]', recipients=['[e-mail]'])
  msg.body = 'This is a test email sent from Flask.'
  mail.send(msg)
  return 'Email sent!'

This Flask code sets up email config, defines a route to send a simple email with subject and body, then sends it using Flask-Mail.

Execution Table

Step	Action	Evaluation	Result
1	Start Flask app	App initialized with mail config	Ready to accept requests
2	User visits /send route	Route function send_email() called	Preparing email message
3	Create Message object	Subject='Hello', sender and recipients set	Message object ready
4	Set message body	Body='This is a test email sent from Flask.'	Message content complete
5	Call mail.send(msg)	Connect to SMTP server with config	SMTP connection established
6	Send email via SMTP	Email data sent to server	Email delivered to recipient server
7	Close SMTP connection	Connection closed cleanly	Resources freed
8	Return response	Return 'Email sent!'	User sees confirmation message

💡 Email sent successfully and response returned to user

Variable Tracker

Variable	Start	After Step 3	After Step 4	After Step 6	Final
app	Flask instance	Configured with mail settings	Same	Same	Same
msg	None	Message object with subject/sender/recipients	Body added	Same	Same
mail	Mail instance	Same	Same	Used to send email	Same

Key Moments - 3 Insights

Why do we need to configure MAIL_SERVER and MAIL_PORT before sending?

Because the app needs to know which SMTP server to connect to for sending emails, as shown in step 5 of the execution_table.

What happens if the recipients list is empty?

The mail.send(msg) call will fail because there is no destination for the email, so no email is sent (refer to step 6).

Why do we set msg.body separately after creating the Message object?

Because the Message constructor sets basic info like subject and recipients, but the body text is added afterward to complete the message content (see steps 3 and 4).

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'msg' after Step 4?

A Message object with subject, sender, recipients, and body set

B Message object with only subject and sender set

C Empty Message object

D Message object with body but no recipients

💡 Hint

Check the 'Evaluation' and 'Result' columns at Step 4 in the execution_table

At which step does the app establish connection to the SMTP server?

A Step 3

B Step 6

C Step 5

D Step 7

💡 Hint

Look for 'Connect to SMTP server' in the 'Evaluation' column in the execution_table

If MAIL_USERNAME is incorrect, what will most likely happen during execution?

A Email sends successfully anyway

B SMTP connection fails at Step 5

C Message object creation fails at Step 3

D Response 'Email sent!' is returned immediately

💡 Hint

Refer to Step 5 where SMTP connection is established using MAIL_USERNAME

Concept Snapshot

Flask-Mail sends emails by:
1. Configuring MAIL_SERVER, PORT, USERNAME, PASSWORD
2. Creating a Message with subject, sender, recipients
3. Setting message body
4. Calling mail.send(msg) to connect SMTP and send
5. Returning response after sending
Use correct SMTP config to avoid connection errors.

Full Transcript

This visual execution trace shows how a Flask app sends a simple email using Flask-Mail. The app starts with mail server configuration. When a user visits the /send route, the app creates a Message object with subject, sender, and recipients. Then it sets the email body text. Next, the app connects to the SMTP server using the configured settings and sends the email. After sending, it closes the connection and returns a confirmation message to the user. Variables like 'msg' and 'mail' hold the message and mail handler objects, changing as the email is prepared and sent. Key points include the need for correct SMTP settings and the order of creating and sending the message. The quiz questions help check understanding of message state, SMTP connection step, and error cases.