Content-Type: multipart/alternative; boundary="..."

Email Tutorials - Herong's Tutorial Examples

∟Content-Type: multipart/alternative; boundary="..."

This section provides an email message example with 'Content-Type: multipart/alternative; ...' header to specify that the content contains multiple alternative parts.

What Is 'Content-Type: multipart/alternative; boundary="..."' Email Header? "Content-Type: multipart/alternative" specifies that the email message content contains multiple parts separated by the given boundary line. Each part is considered as an alternative to the other parts. "multipart/alternative" allows the email client to present the most suitable alternative part to the email recipient.

The following email message contains a content of 2 alternative parts: one with the default of "Content-Type: text/plain". the other with "Content-Type: text/html".

Message-Id: <20231113111946.5EABD266002B@mail.herongyang.com>
Date: 15 Nov 2023 06:19:34 -0500 (EST)
From: chloe@herongyang.com
Content-Type: multipart/alternative; boundary="___S7sFgzbbueqa___"

This is a multi-part message in MIME format.
--___S7sFgzbbueqa___

Chloe & Oliver

INVITE YOU TO CELEBRATE THEIR WEDDING DAY

JULY 7, 2027
--___S7sFgzbbueqa___
Content-Type: text/html

<p style="text-align: center">
<span style="font-family: 'Dancing Script'; font-size: 4em">Chloe
<span style="font-size: 0.5em">&</span> Oliver</span><br/>
<br/>
<span style="font-family: sans-serif; font-size: 1em">
INVITE YOU TO CELEBRATE THEIR WEDDING DAY<br/>
<br/>
<span style="font-size: 2em">JULY 7, 2027</span></span>
<p>
--___S7sFgzbbueqa___--

If a text based email tool receives the above message, it will select the first part as the message content to display.

If a Web browser based email tool receives the above message, it will select the second part as the message content and render it as a nicely formatted page.

Note that the boundary string specified in the header is used to identify the beginning end ending of each part in the message content as described below:

The first occurrence of a line starting with 2 hyphens (--) and followed by the boundary string indicates the beginning of the first part.
Subsequent lines starting with with 2 hyphens (--) and followed by the boundary string indicates the ending of the previous part and the beginning of the next part.
The last occurrence of a line starting with 2 hyphens (--), followed by the boundary string and 2 hyphens (--) indicates the ending of the last part.