Flask-Mail¶

One of the most basic functions in a web application is the ability to send emails to your users.

The Flask-Mail extension provides a simple interface to set up SMTP with your Flask application and to send messages from your views and scripts.

Installing¶

Install from PyPI using an installer such as pip:

$ pip install Flask-Mail

Configuring¶

Flask-Mail is configured through the standard Flask config API. These are the available options (each is explained later in the documentation):

flask-mail.MAIL_SERVER: str = localhost¶

flask-mail.MAIL_PORT: int = 25¶

flask-mail.MAIL_USE_TLS: bool = False¶

flask-mail.MAIL_USE_SSL: bool = False¶

flask-mail.MAIL_DEBUG: bool = app.debug¶

flask-mail.MAIL_USERNAME: str | None = None¶

flask-mail.MAIL_PASSWORD: str | None = None¶

flask-mail.MAIL_DEFAULT_SENDER: str | None = None¶

flask-mail.MAIL_MAX_EMAILS: int | None = None¶

flask-mail.MAIL_SUPPRESS_SEND: bool = app.testing¶

flask-mail.MAIL_ASCII_ATTACHMENTS: bool = False¶

In addition, the standard Flask TESTING configuration option is used by Flask-Mail in unit tests (see below).

The extension instance¶

Emails are managed through a Mail instance:

from flask import Flask
from flask_mail import Mail

app = Flask(__name__)
mail = Mail(app)

In this case all emails are sent using the configuration values of the application that was passed to the Mail class constructor.

Alternatively you can set up your Mail instance later at configuration time, using the init_app() method:

mail = Mail()

app = Flask(__name__)
mail.init_app(app)

In this case emails will be sent using the configuration values from Flask’s current_app context global. This is useful if you have multiple applications running in the same process but with different configuration options.

Load email configuration

Note that Flask-Mail needs the configuration parameters to create a mail handler, so you have to make sure to load your configuration before the initialization of Flask-Mail (either using Mail constructor or init_app() method).

Sending messages¶

To send a message first create a Message instance:

from flask_mail import Message

@app.route("/")
def index():
    msg = Message(
        subject="Hello",
        sender="from@example.com",
        recipients=["to@example.com"],
    )

You can set the recipient emails immediately, or individually:

msg.recipients = ["you@example.com"]
msg.add_recipient("somebodyelse@example.com")

If you have set MAIL_DEFAULT_SENDER you don’t need to set the message sender explicity, as it will use this configuration value by default:

msg = Message(
    subject="Hello",
    recipients=["to@example.com"],
)

If the sender is a two-element tuple, this will be split into name and address:

msg = Message(
    subject="Hello",
    sender=("Me", "me@example.com"),
)
assert msg.sender == "Me <me@example.com>"

The message can contain a body and/or HTML:

msg.body = "testing"
msg.html = "<b>testing</b>"

Finally, to send the message, you use the Mail instance configured with your Flask application:

mail.send(msg)

Bulk emails¶

Usually in a web application you will be sending one or two emails per request. In certain situations you might want to be able to send perhaps dozens or hundreds of emails in a single batch - probably in an external process such as a command-line script or cronjob.

In that case you do things slightly differently:

with mail.connect() as conn:
    for user in users:
        msg = Message(
            subject=f"hello, {user.name}",
            body="...",
            recipients=[user.email],
        )
        conn.send(msg)

The connection to your email host is kept alive and closed automatically once all the messages have been sent.

Some mail servers set a limit on the number of emails sent in a single connection. You can set the max amount of emails to send before reconnecting by specifying the MAIL_MAX_EMAILS setting.

Attachments¶

Adding attachments is straightforward:

with app.open_resource("image.png") as fp:
    msg.attach("image.png", "image/png", fp.read())

If MAIL_ASCII_ATTACHMENTS is set to True, filenames will be converted to an ASCII equivalent. This can be useful when using a mail relay that modify mail content and mess up Content-Disposition specification when filenames are UTF-8 encoded. The conversion to ASCII is a basic removal of non-ASCII characters. It should be fine for any unicode character that can be decomposed by NFKD into one or more ASCII characters. If you need romanization/transliteration (i.e ß → ss) then your application should do it and pass a proper ASCII string.

Unit tests and suppressing emails¶

When you are sending messages inside unit tests, or in a development environment, it’s useful to be able to suppress email sending.

If the setting TESTING is set to True, emails will be suppressed. Calling Message.send() will not result in any messages being actually sent.

Alternatively outside a testing environment you can set MAIL_SUPPRESS_SEND to True. This will have the same effect.

However, it’s still useful to keep track of emails that would have been sent when you are writing unit tests.

In order to keep track of dispatched emails, use the record_messages() method:

with mail.record_messages() as outbox:
    mail.send_message(
        subject="testing",
        body="test",
        recipients=emails,
    )
    assert len(outbox) == 1
    assert outbox[0].subject == "testing"

The outbox is a list of Message instances sent.

Header injection¶

To prevent header injection, attempts to send a message with newlines in the subject, sender or recipient addresses will result in a BadHeaderError.

Signalling support¶

Flask-Mail provides signalling support through a email_dispatched signal. This is sent whenever an email is dispatched (even if the email is not actually sent, i.e. in a testing environment).

A function connecting to the email_dispatched signal is sent with the Flask instance as the first argument, and the Message} instance as the message argument.

def log_message(app, message):
    app.logger.debug(message.subject)

email_dispatched.connect(log_message)