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Python

Send​

import os
import requests

response = requests.post(
    "https://api.epostix.com/v1/emails",
    headers={
        "Authorization": f"Bearer {os.environ['EPOSTIX_API_KEY']}",
        "Idempotency-Key": f"order-{order.id}-confirmation",
    },
    json={
        "from": "[email protected]",
        "to": ["[email protected]"],
        "subject": "It works",
        "html": "<p>Sent from Python.</p>",
    },
    timeout=30,
)

if not response.ok:
    error = response.json()
    raise RuntimeError(f"{error['type']}: {error['message']} ({error['request_id']})")

print(response.json()["id"])

Always pass timeout. requests has none by default, so a stalled connection hangs the worker until something else kills it.

Django​

If you already have send_mail calls, repoint the backend and they keep working:

# settings.py
EMAIL_BACKEND = "django.core.mail.backends.smtp.EmailBackend"
EMAIL_HOST = "smtp.epostix.com"
EMAIL_PORT = 587
EMAIL_USE_TLS = True
EMAIL_HOST_USER = os.environ["EPOSTIX_SMTP_USER"]
EMAIL_HOST_PASSWORD = os.environ["EPOSTIX_SMTP_PASSWORD"]
DEFAULT_FROM_EMAIL = "[email protected]"

Use the API for anything SMTP cannot express: tags, metadata, scheduling, idempotency, and reading the delivery timeline back.

Celery tasks need idempotency keys​

A Celery task can run more than once. Acks-late, a worker restart, or a retry all redeliver it, and without a key each attempt sends another email.

@shared_task(bind=True, max_retries=3)
def send_receipt(self, order_id):
    order = Order.objects.get(pk=order_id)

    response = requests.post(
        "https://api.epostix.com/v1/emails",
        headers={
            "Authorization": f"Bearer {settings.EPOSTIX_API_KEY}",
            "Idempotency-Key": f"order-{order.id}-receipt",
        },
        json={...},
        timeout=30,
    )

    if response.status_code >= 500:
        raise self.retry(countdown=2 ** self.request.retries)

Derive the key from the order, never from uuid4() at call time. A fresh key on the retry sends a second receipt, which is exactly what you were trying to avoid.

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