Definition
MIME (Multipurpose Internet Mail Extensions) is the internet standard that transformed email from a plain-text-only medium into the rich, multimedia-capable communication channel used today. Originally defined in 1992, MIME specifies how email messages can include HTML formatting, attached files, images, non-ASCII character sets, and multiple content alternatives within a single message.
Every modern email relies on MIME. When a subscriber opens an HTML email with images, branding, and formatting, they are seeing the result of MIME encoding.
What MIME Enables
- HTML content: Emails can include styled text, colours, fonts, and layout markup
- File attachments: Documents, images, PDFs, and other files can be attached to messages
- Multipart messages: A single email can contain both HTML and plain-text versions, letting the email client display the most appropriate format
- Non-ASCII text: Emails can include characters from any language through character encoding support
- Inline images: Images can be displayed within the email body rather than as separate attachments
Common MIME Types in Email
| MIME Type | Purpose |
|---|---|
| text/plain | Plain text version of the email |
| text/html | HTML formatted version of the email |
| multipart/alternative | Container for plain text and HTML versions |
| multipart/mixed | Container for message body plus attachments |
| image/jpeg, image/png, image/gif | Embedded or attached images |
| application/pdf | PDF attachments |
How MIME Multipart Messages Work
Most marketing emails use multipart/alternative with two parts:
- A plain text version (
text/plain) — used by older email clients, screen readers, and when HTML is blocked - An HTML version (
text/html) — used by modern email clients for the visual experience
The email client automatically selects which version to display. If the client supports HTML, it shows the HTML version. If not, it falls back to plain text.
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="boundary123"
--boundary123
Content-Type: text/plain
This is the plain text version of the email.
--boundary123
Content-Type: text/html
<p>This is the <strong>HTML</strong> version.</p>
--boundary123--
Why MIME Matters for Email Marketing
- Deliverability: Emails with missing or incorrect MIME headers are more likely to be flagged as spam.
- Rendering consistency: Correct MIME structure ensures emails render properly across different email clients.
- Plain text fallback: A well-formed MIME multipart message ensures subscribers who block HTML or use screen readers still receive the message content.
- File size management: MIME encoding adds approximately 33% overhead to attached binary files through Base64 encoding.
Related Glossary Terms
A/B Testing
A/B testing in email marketing is the practice of sending two variations of an email to a small sample of your list to determine which version performs better before sending the winner to the remaining subscribers.
Abandoned Cart Email
An abandoned cart email is an automated message sent to customers who added items to their online shopping cart but left without completing the purchase. It is one of the highest-converting email types in ecommerce.
AIDA Model for Email
The AIDA model (Attention, Interest, Desire, Action) is a classic copywriting framework used to structure email campaigns that guide subscribers from awareness to conversion.
AMP for Email
AMP for Email is a Google-developed framework that allows email messages to include interactive elements like forms, carousels, accordions, and live content. It turns static emails into dynamic, interactive experiences directly inside the inbox.
Announcement Email
An announcement email is a dedicated campaign that communicates a specific update, milestone, or change to subscribers, from product launches and feature releases to company news and events.
AOL Mail for Email Marketers
AOL Mail is a legacy email provider with specific deliverability requirements and rendering quirks, now operating as part of the Yahoo+AOL network under shared infrastructure.
Frequently Asked Questions
Emails without MIME headers are treated as plain text only. The email client will not interpret HTML tags or render formatting. Most email marketing platforms automatically add proper MIME headers, but custom-coded or API-sent emails may require manual header configuration.
SMTP is the protocol that transports email between servers. MIME is the standard that defines the structure and content of the email message itself. SMTP handles the delivery. MIME handles what is being delivered. Both are required for modern email to function.
Yes. MIME uses Base64 encoding to convert binary attachment data into text format, which increases the original file size by approximately 33%. A 1MB PDF attachment becomes roughly 1.33MB after MIME encoding. This matters for email size limits (typically 10-25MB total).
The `MIME-Version: 1.0` header tells email clients which version of the MIME standard the message uses. It must be included in every MIME-formatted email. Without it, email clients may not parse the multipart structure correctly.
Custom MIME types are not recommended for email because most email clients do not recognise or handle them. Stick to standard MIME types: `text/plain`, `text/html`, `multipart/alternative`, `multipart/mixed`, and common image and document types.