What are MIME types?

You've probably never had to think about MIME types when building a campaign. Your ESP handles them quietly in the background. But when something breaks (say, your beautifully designed email renders as a wall of raw HTML code), MIME types are usually the culprit.

MIME types tell email clients what kind of content they're looking at and how to handle it. They follow a simple type/subtype pattern. The ones you'll actually encounter in email:

• text/plain. Plain text, no formatting
• text/html. The HTML version of your email
• multipart/alternative. A message that contains both a plain text and an HTML version (this is how most well-built emails are structured)
• multipart/mixed. A message with attachments alongside body content
• image/png, image/jpeg, image/gif. Embedded or attached images
• application/pdf. PDF attachments
• application/octet-stream. A generic binary file when no specific type is declared

MIME types live inside the Content-Type header of each part of your message. A typical HTML email header looks like this:

Content-Type: text/html; charset=UTF-8

And a multipart message (the kind that includes both HTML and plain text) looks like:

Content-Type: multipart/alternative; boundary="----=_Part_123"

That boundary value is how the client knows where one part ends and another begins.

Where this matters practically: if your HTML email is declared as text/plain by mistake, your subscriber sees raw code instead of your design. If an attachment has the wrong MIME type, the client might not open it correctly, or a security gateway might flag it entirely. (Executable file types like application/x-msdownload will almost always trigger a block or a warning, regardless of what's actually in the file.)

Your ESP sets these automatically, so you're unlikely to misconfigure them unless you're building custom MIME payloads or working directly with an API. If you are doing that and something looks off, our free Email Header Analyzer can show you exactly what content types are declared in a live message.