Send Your Email Marketing Messages as Multipart

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​ © Seth Lemmons; CC BY 2.0 license
by Heinz Tschabitscher
An independent writer who has reviewed hundreds of email programs and services since 1997.
Updated May 15, 2018

​Sending marketing email as multipart/alternative ensures the message is displayed correctly in either HTML or plain text formatting. Should you send your newsletter in plain text and abandon the advantage of rich HTML formatting, or should you risk annoying some who detest HTML more than anything else?

What is multipart/alternative?

Multipart/alternative emails contain both a plain text and an HTML part. Which part is shown to the user is determined by their email client, and (in some cases) by their choice.

If an email client cannot render HTML messages, it will display the plain text version. HTML-enabled email programs will usually show the rich HTML version, but some let the user decide which they prefer. People with vision impairment, for instance, might prefer the plain text version.

With multipart/alternative messages just about everybody gets the best of both worlds, and you do not have to ask subscribers for their preference and maintain two separate subscriber lists or further complicate segmentation.

Is there a downside to multipart/alternative?

The only disadvantage of multipart/alternative messages is their (slightly) larger size, but as network capacities grow both on desktops and through mobile carriers, this is almost negligible.

Send Your Email Marketing Messages as Multipart/Alternative

To have your marketing messages delivered as multipart/alternative emails that display well practically anywhere:

  1. Make sure your email marketing software or service provider supports multipart/alternative messages.
  2. Compose both a rich HTML version of your message and a plain text equivalent.
    1. If your email marketing software or service creates a plain text version automatically, verify its quality before sending.
  3. Send them both together as one multipart/alternative message.

How does multipart/alternative work?

Multipart/alternative emails make use of the MIME email standard. The individual parts are sent similar to attached files, but so that email programs recognize them as alternative versions; instead of showing all the versions one after the other (or possibly as files available for download), only the preferred version should be displayed.

The alternative versions in a multipart/alternative email are separated by a boundary marker, which is identical for all versions.

Each version also has a MIME content type assigned. This is where the versions differ. With multipart/alternative marketing emails, the content types will typically be "text/plain" and "text/html".

The types follow each other, and (unless user preferences specify otherwise), email programs will usually display the last version they are capable of showing. This means "text/plain" followed by "text/html" in multipart/alternative emails.

Multipart/Alternative Example

The source of an email using multipart/alternative formatting could look like this:

From: Sender <sender@example.com>
To: recipient@example.com
Subject: Example
Date: Fri, 13 Nov 2015 19:36:00 +0000 (GMT)
MIME-version: 1.0
Content-type: multipart/alternative;
boundary="Boundary_MA2"
--Boundary_MA2
Content-type: text/plain; CHARSET=US-ASCII; format=flowed
Content-transfer-encoding: 7BIT
This is but a test.
--Boundary_MA2
Content-type: text/html; CHARSET=US-ASCII
Content-transfer-encoding: 7BIT
<html><body><div><em>This</em> is but a test.</div></body></html>
--Boundary_MA2--