I am developing a mail client on Android, so I need decryption mail send from an Outlook client use Certificate to signature and encryption, I have import user certificate on the phone and I have read user's public key and private key. I also read the rfc-1991 about pgp, but I can not decrypt the encryption mail, so I want to know if Outlook encrypt mail flow the pgp specification. I also found that the Samsung device system mail client can decrypt Outlook encryption mail, or is there any docs about how outlook client encrypt mail?
Outlook encrypts and signs email using S/MIME, not OpenPGP (unless you use some OpenPGP/MIME plugin). X.509 certificates are not used in OpenPGP either (they are used in S/MIME).
S/MIME is defined in RFC 5751, yet it makes little sense to re-implement it yourself. There exist various libraries to help you work with S/MIME in Java. For example, our SecureBlackbox has a variant for Android in its Java edition.