Sending SMIME messages with sendMail endpoint is broken.

Describe the bug

Hello,

We are using Microsoft Graph Java SDK to send encrypted SMIME Messages.
We are already encrypting the content and provide a full MIME document to endpoint:
https://graph.microsoft.com/v1.0/me/sendMail

For example our MIME Message looks like this (saved on content variable):

Date: Tue, 10 Feb 2026 11:24:45 +0000 (UTC)
From: HenriettaM@xyz.onmicrosoft.com
To: xxx-test@gmx.de
Message-ID: <....>
Subject: Test2
MIME-Version: 1.0
Content-Type: application/pkcs7-mime; name="smime.p7m"; smime-type=authEnveloped-data
Content-Transfer-Encoding: base64

MIAGCyqGSIb3DQEJEAEXoIAwgAIBADGCAdswggHXAgEAMIGPMHcxCzAJBgNVBAYTAkRFMQwwCgYD
...

We are using the following code snipplet to send the Email:

initializeGraphClient();
String encoded = Base64.getEncoder().encodeToString(content.getBytes());

RequestInformation requestInfo = new RequestInformation();
requestInfo.httpMethod = HttpMethod.POST;
requestInfo.urlTemplate = "{+baseurl}/users/{+email}/sendMail";
requestInfo.pathParameters = new HashMap<>();
requestInfo.pathParameters.put("baseurl", "https://graph.microsoft.com/v1.0");
requestInfo.pathParameters.put("email", config.getMailbox());
requestInfo.headers.put("Content-Type", Set.of("text/plain"));
requestInfo.setContentFromScalar(client.getRequestAdapter(), "text/plain", encoded);

HashMap<String, ParsableFactory< ? extends Parsable>> errorMapping = new HashMap<>();
errorMapping.put("4XX", ODataError::createFromDiscriminatorValue);
errorMapping.put("5XX", ODataError::createFromDiscriminatorValue);

client.getRequestAdapter().sendPrimitive(requestInfo, errorMapping, Void.class);

However our SMIME content is converted into an attachment and send as an attachment packed into a multipart/mixed structure by sending the Email.

Here is what we get on the reciever side:

Content-Type: multipart/mixed;
	boundary="_002_155174533371770722695547_"
MIME-Version: 1.0

--_002_155174533371770722695547_
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: quoted-printable

--_002_155174533371770722695547_
Content-Type: application/pkcs7-mime; name="smime.p7m"
Content-Description: smime.p7m
Content-Disposition: attachment; filename="smime.p7m"; size=4483;
	creation-date="Tue, 10 Feb 2026 11:33:38 GMT";
	modification-date="Tue, 10 Feb 2026 11:33:38 GMT"
Content-ID: <C06558A1F17AA04CAE95309F8C325E89@namprd19.prod.outlook.com>
Content-Transfer-Encoding: base64

MIAGCyqGSIb3DQEJEAEXoIAwgAIBADGCAdswggHXAgEAMIGPMHcxCzAJBgNVBAYTAkRFMQwwCgYD

Can you fix sending a SMIME by using this sendMail endpoint or is there any alternative ways for sending SMIME with Graph API?
We basically followed the Example 4 in this documentation page:

https://learn.microsoft.com/en-us/graph/api/user-sendmail?view=graph-rest-1.0&tabs=http#example-4-send-a-new-message-using-mime-format

Expected behavior

Sending SMIME messages should work with the sendMail endpoint when Content-Type is provided as text/plain.

How to reproduce

See the code snipplet above for example code.
We can also provide a full SMIME example content if you want.

SDK Version

No response

Latest version known to work for scenario above?

No response

Known Workarounds

No response

Debug output

No response

Configuration

No response

Other information

No response

Start with the sendMail MIME example in the linked Microsoft Graph documentation and reproduce the RequestInformation request shown in the issue, including its Content-Type and encoded body. Compare the request with the received multipart/mixed message to determine whether the behavior belongs to the Java SDK or the Graph endpoint. Done means a confirmed SDK fix, or a documented Graph limitation and supported alternative.