What is a multipart message?

A multipart message is an email that contains the same content in multiple formats, usually plain text and HTML. Your email platform builds both versions into the same message, and the recipient's email client picks whichever one it can display.

Why does this matter? Not every inbox can render HTML. Some corporate email systems strip HTML for security. Some readers prefer plain text for accessibility or speed. And some spam filters actually check whether you included a plain text version (no plain text can be a small red flag).

When you send a multipart message, the structure looks like this (simplified): the email contains a plain text section and an HTML section. Gmail shows the HTML. A text-only client shows the plain text. If you only send HTML and someone's client can't display it, they see nothing (or garbled code).

Most ESPs (Mailchimp, SendGrid, Postmark) automatically generate the plain text version from your HTML, but auto-generated plain text is often terrible. Links turn into raw URLs, formatting disappears, and sometimes you get random line breaks in weird places. If you care about the reader experience (and deliverability), write your own plain text version.

Multipart also covers attachments. If you attach a PDF or image, that's technically another "part" of the message. But when people say "multipart" in deliverability conversations, they usually mean plain text + HTML.

The format behind this is called MIME (Multipurpose Internet Mail Extensions). MIME defines how to bundle multiple content types into one email. You don't need to understand MIME deeply unless you're building your own mail server, but knowing it exists helps if you ever read raw email headers.

If you want to see what your plain text version actually looks like, most ESPs let you preview it before sending. Worth checking once, especially if your emails have complex layouts or tables (tables often turn into unreadable messes in plain text).