SMTP on Nikita Chernyi: Principal Backend Engineer

Real-Time Blackhole List (RBL) and DNS Blocklists (DNSBL) for email antispam

cv@rakshazi.me (Nikita Chernyi) — Tue, 30 Jul 2024 12:56:50 +0300

First published on 2024-07-30.

Buna ziua, dragi cititori! I continue my journey on building Postmoogle (opens in new tab) - an SMTP server with Matrix (opens in new tab) interface we use at etke.cc (opens in new tab) for email communications. After discovering Greylisting, Nolisting, Unlisting, and Other Annoyances, and Forward Confirmed reverse DNS, I decided to dive further into the huge hole of email antispam techniques.

Today’s topic is Real-Time Blackhole List (RBL) and DNS Blocklists (DNSBL).

The Definitive Send Email Guide

cv@rakshazi.me (Nikita Chernyi) — Wed, 15 Feb 2023 17:05:25 +0200

First published on 2023-02-15.

Yā, min’na.

Today you gonna read my Magnum Opus of sending emails. Let’s assume you want to send an email from sender@example.com to recipient@example.net, that guide will cover communication between SMTP servers (MTA to MTA), moreover, for the sake of simplicity the examples will use generic tools instead of some specific programming language snippets.

So, we’re gonna send the following message from sender@example.com to recipient@example.net:

Hi there,
Did it hit the Spam folder?

Here is the high-level algorithm of all actions:

DNS, rDNS, FCrDNS

cv@rakshazi.me (Nikita Chernyi) — Thu, 24 Nov 2022 23:22:13 +0200

First published on 2022-11-24.

Sup dawg, I heard you like DNS, so I put DNS into your DNS so you can query DNS while you querying DNS.

– Tibizx

Yeah, memes from 2008 to explain things from 1996 (opens in new tab).

Anyway, to the topic: what does those acronyms stand for?

DNS

Domain Name System = convert website domain in the browser’s address bar to the actual server’s IP where that website hosted. It’s a common thing, so let’s move to the next one.

The BCC Is a Lie

cv@rakshazi.me (Nikita Chernyi) — Sat, 19 Nov 2022 23:15:50 +0200

First published on 2022-11-19.

Hey, Postmoogle (opens in new tab) gets new features and bug fixes, while I get new knowledge, sometimes rather hilarious one. As you know, that whole blog is about “raskhazi finds out things and tries to tuck them into the head”, so today a new thing “clicked” in my mind - the BCC (blind carbon copy), the field in an email when you want to hide recipients, is not a header.

Graylisting, Nolisting, Unlisting, and Other Annoyances

cv@rakshazi.me (Nikita Chernyi) — Sun, 13 Nov 2022 22:38:40 +0200

First published on 2022-11-13.

Hey,

This post is a bit more about citations from other sources with my comments on the topic because the sources provide a pretty clear description of what each topic is and why it was implemented. From my side, I’d add why I hate each topic but have to reconcile with them. Let’s roll!

greylisting

Greylisting is a new method of blocking significant amounts of spam at the mailserver level, but without resorting to heavyweight statistical analysis or other heuristical (and error-prone) approaches. Consequently, implementations are fairly lightweight, and may even decrease network traffic and processor load on your mailserver.

Mandatory Email Headers or How to Send Email (into Spam)

cv@rakshazi.me (Nikita Chernyi) — Sat, 12 Nov 2022 20:44:59 +0200

First published on 2022-11-12.

Hey, It’s time to compose and send the first valid email message!

First of all, there are several mandatory headers and without any of them your email will be rejected:

MIME-Version - defines “how to parse” the thing you sent. It has a hardcoded value of 1.0
Content-Type - defines “how to read” the email you sent. Usually, values are text/html or text/plain (and charset="UTF-8", because we all ❤️ emojis)
Content-Transfer-Encoding - kinda duplicate of Content-Type’s charset, so value 7bit == ASCII and value 8bit == Unicode
From - defines the sender’s email
To - defines the recipient’s email
Message-Id - defines unique identification of that specific email, usually some random string +@your-server.com. Note the format: <random-string@server-address> (the < and > are mandatory)
Date - defines “when it was sent” in RFC1123 with numeric zone format (go stdlib’s time.RFC1123Z format: Mon, 02 Jan 2006 15:04:05 -0700)

Additionally, there are some headers, mandatory only in case of email threads (if you don’t provide them an email will be valid, but an email thread - will not):

How to validate an email address

cv@rakshazi.me (Nikita Chernyi) — Sat, 12 Nov 2022 00:45:46 +0200

First published on 2022-11-12.

Hey, Email validation is hard. Do you know how to validate an email address for sure? Neither do I, but here is a report on what I found while working on postmoogle (opens in new tab).

So, what’s your wild guess? Regular expressions? Well:

only pretty simple cases - yes.
RFC (opens in new tab)-compliant - no (RFC822 is only a “base” one, there are other RFCs that extend it).
Be sure that email address is valid for sure - also no.

So, how you can really do that?

SMTP components or Why sending emails is a rocket science

cv@rakshazi.me (Nikita Chernyi) — Thu, 10 Nov 2022 15:39:41 +0200

First published on 2022-11-10.

Hey, Wanna know the biggest scam you can get in the email world (apart from “email is a decentralized communication protocol”)?

Simple Mail Transfer Protocol.

Simple, huh? They lied. It’s damn rocket science!

I’m just trying to compile a note about what I found while working on postmoogle (opens in new tab).

Let me just throw some smtp-related tags and see if you can correctly handle at least one of them: MUA, MSA, MTA, MDA.