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zulip/docs/subsystems/email.md

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Email

This page has developer documentation on the Zulip email system. If you're trying to configure your server to send email, you might be looking for our guide to sending outgoing email. If you're trying to configure an email integration to receive incoming email (e.g. so that users can reply to message notification emails via email), you might be interested in our instructions for setting up an email integration.

On to the documentation. Zulip's email system is fairly straightforward, with only a few things you need to know to get started.

  • All email templates are in templates/zerver/emails/. Each email has three template files: <template_prefix>.subject.txt, <template_prefix>.txt, and <template_prefix>.source.html. Email templates, along with all other templates in the templates/ directory, are Jinja2 templates.
  • Most of the CSS and HTML layout for emails is in email_base.html. Note that email has to ship with all of its CSS and HTML, so nothing in static/ is useful for an email. If you're adding new CSS or HTML for an email, there's a decent chance it should go in email_base.html.
  • All email is eventually sent by zerver.lib.send_email.send_email. There are several other functions in zerver.lib.send_email, but all of them eventually call the send_email function. The most interesting one is send_future_email. The ScheduledEmail entries are eventually processed by a supervisor job that runs zerver/management/commands/deliver_email.py.
  • Always use user_profile.delivery_email, not user_profile.email, when passing data into the send_email library. The user_profile.email field may not always be valid.
  • A good way to find a bunch of example email pathways is to git grep for zerver/emails in the zerver/ directory.

One slightly complicated decision you may have to make when adding an email is figuring out how to schedule it. There are 3 ways to schedule email.

  • Send it immediately, in the current Django process, e.g. by calling send_email directly. An example of this is the confirm_registration email.
  • Add it to a queue. An example is the invitation email.
  • Send it (approximately) at a specified time in the future, using send_future_email. An example is the followup_day2 email.

Email takes about a quarter second per email to process and send. Generally speaking, if you're sending just one email, doing it in the current process is fine. If you're sending emails in a loop, you probably want to send it from a queue. Documentation on our queueing system is available here.

Development and testing

All the emails sent in the development environment can be accessed by visiting /emails in the browser. The way that this works is that we've set the email backend (aka what happens when you call the email .send() method in Django) in the development environment to be our custom backend, EmailLogBackEnd. It does the following:

  • Logs any sent emails to var/log/email_content.log. This log is displayed by the /emails endpoint (e.g. http://zulip.zulipdev.com:9991/emails).
  • Print a friendly message on console advertising /emails to make this nice and discoverable.

Testing in a real email client

You can also forward all the emails sent in the development environment to an email account of your choice by clicking on Forward emails to an email account on the /emails page. This feature can be used for testing how the emails gets rendered by actual email clients. This is important because web email clients have limited CSS functionality, autolinkify things, and otherwise mutate the HTML email one can see previewed on /emails.

To do this sort of testing, you need to setup an outgoing SMTP provider. Our production advice for Gmail and transactional email providers are relevant; you can ignore the Gmail warning as Gmail's rate limits are appropriate for this sort of low-volume testing.

Once you have the login credentials of the SMTP provider, since there is not /etc/zulip/settings.py in development, configure it using the following keys in zproject/dev-secrets.conf

  • email_host - SMTP hostname.
  • email_port - SMTP port.
  • email_host_user - Username of the SMTP user
  • email_password - Password of the SMTP user.
  • email_use_tls - Set to true for most providers. Else, don't set any value.

Here is an example of how zproject/dev-secrets.conf might look if you are using Gmail.

email_host = smtp.gmail.com
email_port = 587
email_host_user = username@gmail.com
email_use_tls = true

# This is different from your Gmail password if you have 2FA enabled for your Google account.
# See the configuring Gmail to send email section above for more details
email_password = gmail_password

Notes

  • After changing any HTML email or email_base.html, you need to run scripts/setup/inline_email_css.py for the changes to be reflected in the development environment. The script generates files like templates/zerver/emails/compiled/<template_prefix>.html.

  • Images won't be displayed in a real email client unless you change the base_image_uri used for emails to a public URL such as https://chat.zulip.org/static/images/emails (image links to localhost:9991 aren't allowed by modern email providers). See zproject/email_backends.py for more details.

  • While running the backend test suite, we use django.core.mail.backends.locmem.EmailBackend as the email backend. The locmem backend stores messages in a special attribute of the django.core.mail module, "outbox". The outbox attribute is created when the first message is sent. It’s a list with an EmailMessage instance for each message that would be sent.

Email templates

Zulip's email templates live under templates/zerver/emails. Email templates are a messy problem, because on the one hand, you want nice, readable markup and styling, but on the other, email clients have very limited CSS support and generally require us to inject any CSS we're using in the emails into the email as inline styles. And then you also need both plain-text and HTML emails. We solve these problems using a combination of the premailer library and having two copies of each email (plain-text and HTML).

So for each email, there are two source templates: the .txt version (for plain-text format) as well as a .source.html template. The .txt version is used directly; while the .source.html template is processed by scripts/setup/inline_email_css.py (generating a .html template under templates/zerver/emails/compiled); that tool (powered by premailer) injects the CSS we use for styling our emails (templates/zerver/emails/email.css) into the templates inline.

What this means is that when you're editing emails, you need to run scripts/setup/inline_email_css.py after making changes to see the changes take effect. Our tooling automatically runs this as part of tools/provision and production deployments; but you should bump PROVISION_VERSION when making changes to emails that change test behavior, or other developers will get test failures until they provision.

While this model is great for the markup side, it isn't ideal for translations. The Django translation system works with exact strings, and having different new markup can require translators to re-translate strings, which can result in problems like needing 2 copies of each string (one for plain-text, one for HTML) and/or needing to re-translate a bunch of strings after making a CSS tweak. Re-translating these strings is relatively easy in Transifex, but annoying.

So when writing email templates, we try to translate individual sentences that are shared between the plain-text and HTML content rather than larger blocks that might contain markup; this allows translators to not have to deal with multiple versions of each string in our emails.

One can test whether you did the translating part right by running scripts/setup/inline_email_css.py && manage.py makemessages and then searching for the strings in locale/en/LC_MESSAGES/django.po; if there are multiple copies or they contain CSS colors, you did it wrong.

A final note for translating emails is that strings that are sent to user accounts (where we know the user's language) are higher-priority to translate than things sent to an email address (where we don't). E.g. for password reset emails, it makes sense for the code path for people with an actual account can be tagged for translation, while the code path for the "you don't have an account email" might not be, since we might not know what language to use in the second case.

Future work in this space could be to actually generate the plain-text versions of emails from the .source.html markup, so that we don't need to maintain two copies of each email's text.