What is an attachment disposition?

You're building an email with a logo and a PDF download link. How does the email client know which one to show in the message body and which one to list as a file to download?

That's what Content-Disposition decides. It's a header that tells the recipient's email client how to handle each file attached to the message.

There are two values you'll see:

• inline tells the client to display the file directly inside the email body (usually images, sometimes HTML files).
• attachment tells the client to list the file separately as something the reader can download (PDFs, spreadsheets, documents).

When you set an image as inline, it gets displayed where your HTML says it should appear (using a Content-ID reference). When you set a file as attachment, most email clients show it in a list at the top or bottom of the message with a download icon.

Most email builders (Mailchimp, Brevo, Klaviyo) handle this automatically. You upload a logo for your template, it goes in as inline. You add a PDF link, it goes in as attachment. But if you're building raw MIME messages or debugging why an image isn't loading, this header is what you're checking.

One deliverability note: attachments increase your email's spam score more than inline images. A marketing email with a 5MB PDF attachment will hit spam filters harder than the same email with a link to download that PDF. If you're sending transactional emails (receipts, invoices), attachments are expected and fine. If you're sending marketing campaigns, link to the file instead of attaching it.

Not sure if your images are set to inline correctly? Send yourself a test email and check the raw source with our Email Header Analyzer. It'll show you every Content-Disposition header in the message.