Unable to load S/MIME encrypted message with MimeKit.LoadAsync

I'm having issues parsing an S/MIME encrypted E-mail message using MimeKit's LoadAsync method. I want to parse it to a MimeMessage object to later decrypt it. The method throws an error Failed to parse message headers upon calling. Please see code below:

using var mimeMemoryStream = new MemoryStream(Encoding.UTF8.GetBytes(emailMime), false);
var message = await MimeMessage.LoadAsync(mimeMemoryStream);

emailMime contains a string representation of my message, as shown below (sensitive information has been removed and the encrypted body has been truncated):

From: Sender <sender@example.com>
Content-Type: application/pkcs7-mime; name="smime.p7m"; smime-type="enveloped-data"
Content-Disposition: attachment; filename="smime.p7m"
Content-Transfer-Encoding: base64
MIME-Version: 1.0
Date: Tue, 11 Oct 2022 10:03:20 +0200
Message-ID: <b6d24167-f517-44dc-b21f-bb219cd25e74@mail.dll>
Subject: Encrypted and signed
To: Bob <whatever@example.com>
MIKJpwYJKoZIhvcNAQcDoIKJmDCCiZQCAQAxggHkMIIB4AIBADCBxzCBrjELMAkGA1UEBhMCUEwxHDAaBgNVBAgME0Jp
[truncated]
Tpzn/UAqUMCVY1C/nzU3T0zEcJDxDIb4eaVGr+n6w1Xc2nQs7SOBfXumZ67Ds9fCVeNvk28=

Reading mimeMemoryStream returns the original message, so encoding seems fine.

I also tried sending this message to myself and was able to open this email in Outlook decrypt it using my certificate, so I'm not sure if there's anything wrong with the message itself. I also tested my pipeline with an unecrypted message and that works as well.

Solution

For anyone coming here in the future with a similar issue - I found a fix.

There needs to be an extra line between the headers and the encrypted content (so in my example between To: Bob <whatever@example.com> and the encrypted stuff. I guess without it the parser was interpreting the encrypted data as a header declaration, which explains why it was getting confused there.