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

November 12, 2022 updated September 23, 2026 by Nikita Chernyi in SMTP (RSS)

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):

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!

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