Multi-part MIME Email

An email containing HTML is usually sent by email providers as Multi-part MIME (Multipurpose Internet Mail Extensions), by default. This means two versions of your email are sent to the recipient:  

  • One in Plain Text format.
  • One in HTML format.

One reason for this:
Your recipient may have turned on Plain Text Viewing Only in their email software. If not, then the HTML version will be delivered.

This process usually happens behind the scenes, so you may not even realize it's happening. We provide an example below to better explain this process.

Example of sender email client setting

In the following screenshot (using Thunderbird as the email client), we are sending a Multi-part MIME email:

In the following screenshot, using Thunderbird, we are sending a multipart/alternative email:

The default format setting is Auto-Detect, but we decided to choose Plain and Rich (HTML) Text. This option will send the message as Multipart/Alternative or MIME.

The last two lines are in a red font which will require HTML.

Message Source Details

Two versions of the message are sent out - one in Plain Text and one in HTML.

Let's take a look at the message source which tells us exactly how it was sent:

Let's take a look at the message source which tells us how exactly it was sent:

Recipient View - when HTML by default

This recipient's email software is set up to accept HTML, so the HTML version is displayed.

When the message was received by the recipient, this is how it looked like when the email reader was setup to accept HTML by default.

Recipient View - when Plain Text only

This recipient has chosen to display messages in Plain Text Only. Notice the last two lines are no longer displayed in red.

We decided to display the messages in Plain Text only (which is what some recipients may prefer) and this is how the message looked like:

Why would someone decide to view messages in Plain Text Only?

HTML may contain hidden things like viruses or tracking programs, making it unsafe for your computer.  Although many improvements have been made in delivering HTML emails safely, spammers continue to develop new techniques to overcome the safety barriers.

Some recipients (or their employers) choose to receive all emails in Plain Text Only to avoid all viruses and tracking programs.