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

Originally published on 2022-11-12 at /posts/how-to-send-email-into-spam/; republished here on 2026-09-23 with links updated.

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):

• In-Reply-To - defines Message-Id of the previous email in a thread
• References - an array of all Message-Ids in the thread

Here is an example of a fully valid email

MIME-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8BIT
From: your-email-sender@your-server.com
To: some@example.com
Message-Id: <some-random-string@your-server.com>
Date: Sat, 12 Nov 2022 19:13:27 +0000
Subject: A fully valid email example

That will go straight into the Spam dir of your mailbox!