Handling transient errors

Applications using Anymail need to be prepared to deal with connectivity issues and other transient errors from your ESP’s API (as with any networked API).

Because Django doesn’t have a built-in way to say “try this again in a few moments,” Anymail doesn’t have its own logic to retry network errors. The best way to handle transient ESP errors depends on your Django project:

  • If you already use something like celery or Django channels for background task scheduling, that’s usually the best choice for handling Anymail sends. Queue a task for every send, and wait to mark the task complete until the send succeeds (or repeatedly fails, according to whatever logic makes sense for your app).

  • Another option is the Pinax django-mailer package, which queues and automatically retries failed sends for any Django EmailBackend, including Anymail. django-mailer maintains its send queue in your regular Django DB, which is a simple way to get started but may not scale well for very large volumes of outbound email.

In addition to handling connectivity issues, either of these approaches also has the advantage of moving email sending to a background thread. This is a best practice for sending email from Django, as it allows your web views to respond faster.

Automatic retries

Backends that use requests for network calls can configure its built-in retry functionality. Subclass the Anymail backend and mount instances of HTTPAdapter and Retry configured with your settings on the Session object in create_session().

Automatic retries aren’t a substitute for sending emails in a background thread, they’re a way to simplify your retry logic within the worker. Be aware that retrying read and other failures may result in sending duplicate emails. Requests will only attempt to retry idempotent HTTP verbs by default, you may need to whitelist the verbs used by your backend’s API in allowed_methods to actually get any retries. It can also automatically retry error HTTP status codes for you but you may need to configure status_forcelist with the error HTTP status codes used by your backend provider.

import anymail.backends.mandrill
from django.conf import settings
import requests.adapters

class RetryableMandrillEmailBackend(anymail.backends.mandrill.EmailBackend):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        retry = requests.adapters.Retry(
            allowed_methods=False,  # Retry all HTTP verbs
        self.retryable_adapter = requests.adapters.HTTPAdapter(max_retries=retry)

    def create_session(self):
        session = super().create_session()
        session.mount("https://", self.retryable_adapter)
        return session