What is the P7Z file format?
A .p7z file is a CMS CompressedData message - part of the S/MIME and PKCS #7 family of cryptographic message formats. Unlike its sibling .p7s (detached signature) and .p7m (encrypted or signed message), a .p7z file only compresses its payload using zlib/DEFLATE as specified in RFC 3274. It adds no encryption and no digital signature.
In practice, .p7z files arrive almost exclusively as e-mail attachments named smime.p7z, generated automatically when an S/MIME mail client compresses an outgoing message. A recipient whose client supports S/MIME decompression (Outlook, Apple Mail, Thunderbird) never sees the .p7z file - the client silently unwraps it and displays the message normally. The file surfaces only when the recipient's mail application lacks S/MIME support.
The binary format is DER-encoded ASN.1: a CMS ContentInfo structure wrapping a CompressedData value with CMSVersion 0, a compression algorithm identifier (OID 1.2.840.113549.1.9.16.1.9 for zlib), and the compressed MIME content. When transmitted in e-mail, it is typically base64-armoured. Integrity relies on zlib's built-in Adler-32 check; cryptographic integrity requires an additional SignedData wrapper.
The format is defined in RFC 3274 within the Cryptographic Message Syntax (CMS, RFC 5652) framework, forming part of the S/MIME 4.0 suite (RFC 8551).
Security & safety
RISK: MEDIUMA .p7z is a compressed e-mail and only as safe as its contents: the inner message can carry malicious attachments or phishing links that become active once decompressed, so treat an unexpected smime.p7z like any unsolicited mail and don't open enclosed files you don't trust. Note that .p7z provides NO encryption - it is compression only, so it offers no confidentiality (that's the job of .p7m/encrypted S/MIME). Decompression 'zip-bomb' style abuse is theoretically possible but rare. If a smime.p7z won't open, the usual cause is simply a mail client without S/MIME-compression support, not a threat.
Format details
in a nutshell- Split 7-Zip volume - Some tools and older references label a multi-volume 7-Zip part .p7z; in practice 7-Zip splits are named .7z.001 etc., so genuine p7z volumes are rare.
Programs that open P7Z files
Windows3 apps
Microsoft Outlook
Paid Open the message in Outlook; as an S/MIME-capable client it decompresses smime.p7z automatically and shows the normal e-mail. No add-in needed. 
Mozilla Thunderbird
Open-source Open the message (or import the .eml) in Thunderbird, which supports S/MIME and shows the decompressed message. Free. 
OpenSSL (cms)
Open-source Advanced/manual: 'openssl cms -decompress -in smime.p7z -inform DER -out message.eml' to recover the original message, then open the .eml. 
macOS3 apps
Mozilla Thunderbird
Open-source Free S/MIME-capable client; open the message to read the decompressed content. 
Apple Mail
Built-in Built into macOS; Apple Mail supports S/MIME and decompresses the message automatically. Save the original e-mail with the sender's S/MIME configured.
OpenSSL (cms)
Open-source Terminal: 'openssl cms -decompress -in smime.p7z -inform DER -out message.eml' then open the resulting .eml in Mail. 
Linux2 apps
Mozilla Thunderbird
Open-source Open the message in Thunderbird; it decompresses S/MIME and displays the e-mail.
OpenSSL (cms)
Open-source 'openssl cms -decompress -in smime.p7z -inform DER -out message.eml' to extract the original message for any mail viewer. Technical details
deep spec| Encoding | Binary DER (ASN.1) for standalone files; base64-armoured when embedded in MIME e-mail |
| Byte order | Not applicable - ASN.1 DER is tag-length-value encoding, not endian-dependent |
| Container | PKCS #7 / Cryptographic Message Syntax (CMS) - CompressedData ContentInfo |
| Magic bytes | 30 82 (ASN.1 SEQUENCE with 2-byte length) at offset 0 for DER-encoded files; no fixed signature for base64-armoured MIME form |
| Compression | zlib / DEFLATE (RFC 3274 CMS Compressed Data) |
| Encryption | None - .p7z compresses only; encryption requires .p7m (EnvelopedData) |
| CMS OID | 1.2.840.113549.1.9.16.1.9 (id-smime-ct-compressedData) |
| CMSVersion | 0 (fixed for CompressedData) |
| Integrity | zlib Adler-32 checksum built into the compressed stream; no cryptographic integrity unless wrapped in a SignedData layer (.p7m) |
| Typical size | A few KB to a few MB (one e-mail message with its inline parts and attachments) |
| MIME type | application/pkcs7-mime; standard attachment filename smime.p7z |
| Specification | RFC 3274 (CMS Compressed Data); S/MIME 4.0 per RFC 8551 |
| Delivery model | Auto-generated by S/MIME clients; transparent to capable recipients who never see the raw .p7z attachment |
| Sibling formats | .p7m = encrypted/signed EnvelopedData or SignedData; .p7s = detached signature; .p7c = certificate bundle |
| Released | S/MIME compression defined in RFC 3274 (CMS Compressed Data, 2002) |
| Latest version | S/MIME 4.0 (RFC 8551, 2019); compression per RFC 3274 |
| Open standard | Yes · royalty-free |
| Specification | datatracker.ietf.org |
P7Z conversions
Community Q&A
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