MIME (Multipurpose Internet Mail Extensions)

What is MIME (Multipurpose Internet Mail Extensions)?

MIME (Multipurpose Internet Mail Extensions) is a standard that extends the original format of email messages to support a wide range of content types beyond plain text. Introduced in the early 1990s, MIME allows email to include attachments, HTML content, images, videos, and even encrypted data within the same message.

In simple terms, MIME makes modern email possible. Without it, you could only send plain-text messages. No bold text, no inline images, no PDFs or documents.

Every MIME message contains specific headers that describe the type of content being sent, such as:

These headers tell the receiving email client how to interpret and display the message.

How MIME Works

MIME defines a structure that lets email messages carry multiple parts, each with its own content type and encoding.

When you attach a file or include rich text, your mail client converts the message into MIME format. A single email can contain:

• A text/plain version for older clients
• A text/html version for modern ones
• One or more attachments, such as documents or images

Each section is separated by a unique boundary marker, and each part includes its own Content-Type and Content-Transfer-Encoding headers. For example:

When the message arrives, the recipient’s email client decodes and reconstructs it according to these MIME headers.

The Role of MIME in Email Security

While MIME greatly enhances functionality, it also introduces new security considerations. Malicious actors can exploit MIME to disguise harmful attachments or embed deceptive links in HTML messages.

Security systems and filters must therefore:

• Scan MIME attachments for malware or phishing attempts.
• Validate MIME structure to detect malformed or obfuscated content.
• Enforce policies that block risky file types (like .exe or .js).

MIME headers also interact with DKIM signatures, since DKIM signs the entire message content, any modification to MIME structure after signing will invalidate the DKIM signature, protecting the integrity of attachments and message bodies.

MIME and DMARCeye

While DMARCeye primarily focuses on authentication (SPF, DKIM, and DMARC), proper MIME handling is critical for maintaining the integrity of signed messages. If a gateway or relay modifies MIME boundaries, attachments, or encoding, DKIM verification may fail, causing false negatives in DMARC reports.

DMARCeye helps you identify such inconsistencies by analyzing aggregate data from receiving mail servers. Repeated DKIM failures linked to MIME-altering systems often indicate misconfigured filters or content rewriters in your mail path.

By monitoring these issues, DMARCeye helps ensure that MIME-encoded messages remain both secure and authentic throughout delivery.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.