Content-Transfer-Encoding: BASE64
Content-Disposition: attachment; filename=smime.p7m
Content-Type: application/pkcs7-mime; name=smime.p7m;
 smime-type="enveloped-data"
...
X-MS-Has-Attach:
X-MS-TNEF-Correlator:
x-ms-exchange-transport-fromentityheader: Hosted
...
MIME-Version: 1.0

$ openssl smime -decrypt -in msg.eml  -inkey privatekey.pem  -out decrypted.txt
Error decrypting PKCS#7 structure
140735747326920:error:06065064:digital envelope routines:EVP_DecryptFinal_ex:bad decrypt:/BuildRoot/Library/Caches/com.apple.xbs/Sources/libressl/libressl-22.50.2/libressl/crypto/evp/evp_enc.c:529: