How to read S/MIME mails and their attachements

I have a Java program which, with the Microsoft Graph API SDK, reads emails and their attachments from a mailbox, but when someone sends a secure email, it cannot retrieve the content, otherwise if it It's just a secure attachment, I can't recover it either.

How should I manage these emails?

Note : Unfortunately, the Microsoft Graph API does not provide direct support for decrypting S/MIME encrypted emails. While it can retrieve the encrypted content, decryption must be handled separately.

• You need to perform the decryption on the client side using the appropriate cryptographic libraries, as the email body or attachments will remain encrypted when accessed through the API.
• Microsoft Graph API can retrieve encrypted emails and attachments but does not decrypt them.

Fetch email messages, including S/MIME encrypted ones, using Microsoft Graph API:

Get the message ID:

https://graph.microsoft.com/v1.0/me/messages

Get MIME content of an Outlook message:

GET https://graph.microsoft.com/v1.0/me/messages/MessageID/$value

GraphServiceClient graphClient = new GraphServiceClient(requestAdapter);

graphClient.me().messages().byMessageId("{message-id}").content().get();

Get attachment ID:

https://graph.microsoft.com/v1.0/me/messages/MessageID/attachments

Get MIME content of an Outlook message attached to an Outlook item:

https://graph.microsoft.com/v1.0/me/messages/MessageID/attachments/AttachmentID/$value

• Microsoft Graph does not manage the decryption of S/MIME-encrypted email content or attachments.
• As S/MIME encryption depends on the recipient's private key for decryption, you must ensure that your application has access to the correct private keys (for the recipient) in order to decrypt the message content.

Hence, as a workaround you can Retrieve the email and attachments from the Graph API and Decrypt the content and attachments using a cryptographic library in your Java application, such as BouncyCastle or Java's built-in S/MIME support.

For sample, Decrypting an S/MIME email with BouncyCastle:

// Load the encrypted S/MIME message
MimeMessage encryptedMessage = new MimeMessage(session, encryptedInputStream);

// Load the private key from a key store (e.g., PKCS12)
KeyStore keyStore = KeyStore.getInstance("PKCS12");
keyStore.load(new FileInputStream("keystore.p12"), "password".toCharArray());
PrivateKey privateKey = (PrivateKey) keyStore.getKey("privatekeyAlias", "password".toCharArray());

// Decrypt the S/MIME message
SMIMEEnveloped enveloped = new SMIMEEnveloped(encryptedMessage);
RecipientInformationStore recipients = enveloped.getRecipientInfos();
RecipientInformation recipient = recipients.getRecipients().iterator().next();

// Decrypt the content stream
InputStream decryptedContentStream = recipient.getContentStream(privateKey);

• You must decrypt the message content and attachments yourself using the private key and an S/MIME decryption library.
• Libraries like BouncyCastle (Java) or JavaMail can help with decrypting S/MIME messages.

Reference:

Get MIME content of a message using the Outlook mail API - Microsoft Graph | Microsoft