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

This section provides an email message example with 'Content-Type: multipart/mixed; ...' header to indicate that the content contains multiple parts. This is commonly used to send emails with attachments.

What Is 'Content-Type: multipart/mixed; boundary="..."' Email Header? "Content-Type: multipart/mixed" specifies that the email message content contains multiple parts separated by the given boundary line.

The most common use case of the "Content-Type: multipart/mixed" header is for sending emails with attachments. Here is an example:

Return-Path: <>
Received: from (localhost [])
  by (Postfix) with ESMTP id 5EABD266002B
  for <herong>; 15 Nov 2023 06:19:34 -0500 (EST)
Message-Id: <>
Date: 15 Nov 2023 06:19:34 -0500 (EST)
Content-Type: multipart/mixed; boundary="___S7sFgzbbueqa___"

This is a multi-part message in MIME format.

Joe, Attached is my virtual contact file. -- Herong
Content-Type: text/x-vcard
Content-Disposition: attachment; filename="herong.vcf"

FN:Herong Yang

Note that the second part in the above email message uses the 'Content-Disposition: attachment; filename="herong.vcf"' header to indicate that this part should be treated as an attachment file.

Table of Contents

 About This Book

 Introduction to Email

 Postfix - Mail Transport Agent (MTA)

 SSL/TLS Secure Connections with Postfix Server

 Dovecot - IMAP and POP3 Server

 SSL/TLS Secure Connections with Dovecot Server

 Email Client Tools - Mail User Agents (MUA)

 Mozilla Thunderbird - Mail User Agents (MUA)

 PHPMailer - PHP Package for Sending Emails

Email Message Format and Headers

 Email Message Format and Example

 What Are Email Message Headers

 Content-Type: text/html - HTML Content

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

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

 Content-Transfer-Encoding: 8bit - Non-ASCII Content

 Content-Transfer-Encoding: base64 - Base64 Encoded Content

 Content-Transfer-Encoding: quoted-printable - 7-Bit Encoding

 Subject: =?...?= - 7-Bit Encoding

 From: ... - Email "From" Address

 To: ... - Email "To" Addresses

 Received: from ... by ... - SMTP Server Log


 Full Version in PDF/EPUB