Should I use port 587 or 465 in Python smtplib?

Port 587 with server.starttls() is the modern standard (RFC 6409). Port 465 uses implicit SSL (smtplib.SMTP_SSL). Both are secure; CentricSell Mail supports both 587 and 465.

How do I prevent smtplib from blocking my FastAPI web server?

Never call smtplib.SMTP synchronously in an async route. Run it inside a background task (FastAPI BackgroundTasks), a thread pool (asyncio.to_thread), or an asynchronous queue like Celery or RQ.

Send Transactional Email in Python: smtplib & TLS Guide

Executive Key Takeaways

•Always send MIME multipart/alternative emails with both HTML and plain-text versions to maximize deliverability and accessibility.

•Use port 587 with STARTTLS or port 465 with SSL/TLS; never transmit SMTP credentials over unencrypted port 25.

•In high-concurrency Python applications (FastAPI, Celery, Django), reuse SMTP connections or dispatch tasks asynchronously to avoid blocking the event loop.

•CentricSell Mail provides a drop-in ESMTP relay on port 587 that works with standard smtplib without installing third-party SDKs.

1. Python smtplib Fundamentals

Python includes robust email capabilities in its standard library through smtplib and email.message. You don't need heavy third-party vendor SDKs to send reliable transactional email.

A production SMTP client must always fulfill three core requirements: authenticated login (AUTH LOGIN or AUTH PLAIN), encrypted transport (STARTTLS or SSL), and properly formatted RFC 5322 MIME messages.

2. Building Modern HTML & Plaintext MIME Messages

Spam filters heavily penalize emails that contain only HTML without a corresponding plain-text alternative. Use Python 3.6+'s modern EmailMessage class to construct multipart messages seamlessly:

3. Complete Production-Ready Python Script

import smtplib
from email.message import EmailMessage

def send_transactional_email(recipient: str, subject: str, html_body: str, text_body: str):
    msg = EmailMessage()
    msg["Subject"] = subject
    msg["From"] = "CentricSell Mail <notifications@yourdomain.com>"
    msg["To"] = recipient

    # Set plain text fallback first
    msg.set_content(text_body)
    # Add HTML alternative
    msg.add_alternative(html_body, subtype="html")

    # Connect to CentricSell Mail SMTP relay
    with smtplib.SMTP("smtp.centricsell.com", 587, timeout=15) as server:
        server.starttls()
        server.login("your-api-key-id", "your-api-key-secret")
        server.send_message(msg)

# Usage
send_transactional_email(
    recipient="user@example.com",
    subject="Your Secure Verification Code",
    html_body="<h1>Welcome!</h1><p>Your login code is <strong>849201</strong>.</p>",
    text_body="Welcome! Your login code is 849201."
)

4. Async & Background Dispatch in FastAPI and Celery

Because SMTP connections involve a network handshake, TLS negotiation, and data transmission (typically taking 200-800ms), you should always dispatch sends in the background:

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

@app.post("/signup")
async def signup(email: str, background_tasks: BackgroundTasks):
    # Process user signup...
    background_tasks.add_task(
        send_transactional_email,
        recipient=email,
        subject="Welcome to our platform",
        html_body="<p>Account confirmed!</p>",
        text_body="Account confirmed!"
    )
    return {"status": "ok"}

5. Robust Error Handling (SMTPException, SMTPServerDisconnected)

Wrap your SMTP sends with proper exception handling to catch transient network drops and provider throttles:

smtplib.SMTPAuthenticationError: Invalid API key or credentials.
smtplib.SMTPRecipientsRefused: Recipient address rejected (add to suppression list).
smtplib.SMTPServerDisconnected: Connection timed out; safe to retry.

How to Send Transactional Email in Python with smtplib, STARTTLS & Failover (2026)

Executive Key Takeaways

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