What is the MME file format?
An .mme file is a MIME-encoded email message stored as a plain-text file. It follows RFC 2822 header conventions and the RFC 2045-2049 MIME standard: headers appear first (MIME-Version: 1.0, Content-Type:, Subject:, From:), followed by boundary-delimited body parts that each carry their own Content-Type and Content-Transfer-Encoding declarations.
Binary attachments - images, documents, executables - are embedded as Base64-encoded blocks within the text envelope, which expands the original binary data by approximately one-third. Text parts near ASCII use quoted-printable encoding instead.
The .mme extension was used by early AOL software and certain older email clients in the 1990s. It is functionally identical to .eml and .mim files; all three share the same RFC 2822 structure. Modern email clients default to .eml when saving MIME messages.
Because the file is ASCII text, any text editor can open and read it. To open an .mme in a modern email client, rename it to .eml - Outlook, Thunderbird, and Apple Mail all accept .eml directly. The body text is human-readable; only Base64-encoded attachment blocks appear garbled in a raw text view.
S/MIME extensions can add CMS/PKCS#7 digital signatures or encryption to individual MIME parts, but the base .mme format itself provides no integrity or confidentiality guarantees.
Security & safety
RISK: MEDIUMMIME files are email messages and can contain malicious attachments (executables, macro-enabled Office documents, etc.) embedded as Base64. Opening the .mme as text is safe; extracting and running attachments carries the same risk as any email attachment. Phishing emails may be distributed as .mme files. Do not run extracted attachments from unknown senders.
Format details
in a nutshell- WinZip MIME-encoded archive - WinZip can open .mme as a MIME-encoded container to extract attachments, treating it as an encoded archive wrapper.
Programs that open MME files
Windows5 apps
Mozilla Thunderbird
Open-source Rename .mme to .eml; drag into Thunderbird or File → Open Saved Message. 
Rename to .eml + Windows Mail / Outlook
Built-in Rename file.mme to file.eml, then double-click to open in Windows Mail or Outlook. 
macOS2 apps
Technical details
deep spec| Container standard | MIME (RFC 2045-2049) over RFC 2822 email headers |
| Character encoding | 7-bit ASCII text envelope; binary content encoded as Base64 or quoted-printable |
| MIME type | message/rfc822 (also application/mime, text/plain) |
| Version header | `MIME-Version: 1.0` - mandatory first MIME header in the file |
| Part delimiter | Multipart boundary string declared in `Content-Type: multipart/*; boundary="..."` |
| Binary encoding overhead | Base64 expands binary attachments by approximately 33% over original binary size |
| Signature/encryption | S/MIME (CMS/PKCS#7) can add per-part digital signatures or encryption |
| Functional equivalents | `.eml` and `.mim` are structurally identical; rename `.mme` to `.eml` to open in modern clients |
| Historical origin | `.mme` extension used by early AOL software (1990s); largely superseded by `.eml` |
| Typical file size | 5 KB - 25 MB (Base64 expansion inflates attachment sizes by ~33%) |
| Human-readable body | Message body is plain ASCII; only Base64-encoded attachment blocks appear garbled |
| Built-in integrity | None in base format; S/MIME adds optional digital signatures |
| Released | 1992 (MIME standard, RFC 1341); .mme extension used by early AOL and email clients |
| Latest version | RFC 2045-2049 (1996, MIME); email format unchanged at core |
| Open standard | Yes · royalty-free |
| Specification | datatracker.ietf.org |
MME conversions
Community Q&A
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