Outlook receiving signed emails as attachment

Question

Hi, I'm actually getting to this problem from the other side but trying to understand why this is happening:]

My outgoing emails (from the server of the provider of my domain and also the ones from gmails) signed with valid certificate
arrive as normal body emails with the signed security certifications to any accounts that is not run by outlook. But if I send them to an account that is based on outlook they receive them as attachment.
Quite frustrating that I try to increase the security and trust of my emails only to look like a fool when I send something to an Outlook email. How is it possible that on the year 2021 Outlook can't cope with receiving emails with signed certificates? What's the issue? Why Outlook can't properly show signed emails?

Thanks

Answer 1

We are having the same issue.

I don't know if you ever got an answer to your query, but one thing I did notice from your screenshots, is that the example sent to you from an Outlook user, has a string in the message body about it being an external message. This is probably appended as a transport rule, as we do something similar to external emails, just as a notice to users.

My suspicion is that, as you have signed the message, Exchange moves your message into an attachment so that when the transport rule appends some text to the body, it does not impact the signature of your message.

Answer 2

Hello @Davide Zerilli ,

Can you send a screen snapshot (redacted as necessary) that shows the problem?

I have received and sent signed messages with Outlook for at least 10 years now and it works. In a "summary" view of a folder, signed messages are shown as having an attachment (even when they don't have a "normal" attachment) but the messages display OK when previewed or opened.

The reason that an attachment is shown is because the message is sent with a content type like:

Content-Type: multipart/signed;  
        protocol="application/x-pkcs7-signature";  
        micalg=SHA1;  
        boundary="----=_NextPart_000_0019_01D7EAED.9A2406C0"  

At least on of the parts has these headers:

Content-Type: application/pkcs7-signature;  
        name="smime.p7s"  
Content-Transfer-Encoding: base64  
Content-Disposition: attachment;  
        filename="smime.p7s"  

Gary

Answer 3

I found this when I ran into the same problem. I discovered, using the Outlook application in Windows at least, that if I open the attachment, I can reply or reply all from there, and that allows me to thread the conversation. Hope this helps.