SMIME signature as attachment if sent over Azure Communication Service

Question

Until now we used Microsoft Exchange Online to send our emails to our customers. Since the volume of emails increased over the years and we start hitting the limits of Exchange Online we set up a Azure Communication Service to send emails over it. This works good so far but we have this issue with our SMIME signature. Our application signs all outgoing email with a certificate, and when the application sends it over Microsoft Exchange Online to a gmail address, you can see the "verified email address" banner in the email:

But if we send it over Azure Communication Service (using SMTP not the API), the signature banner doesn't show but the signature is added as an attachment:

Does anyone know why that happens and how we can change that? The only thing I did in our application is changing the SMTP configuration.

Cheers!

Answer 1

Hi @Alessandro Petrillo

It sounds like the problem is caused by the difference in how Microsoft Exchange Online and Azure Communication Services handle S/MIME signatures. When you use Exchange Online, the service may correctly detect and process S/MIME signatures to display a "Verified email address" banner in Gmail. However, when you switch to Azure Communication Services, the S/MIME signature doesn't seem to be interpreted correctly, causing it to appear as an attachment instead of a verified signature.

Considering that your issue is mostly focused on Azure Communication Services, and I'm only focusing on Exchange and mail flow, I'll just give a few superficial suggestions:

1. Test sending the signed email to an email client other than Gmail to see if the problem persists. This may help determine if the issue is specific to how Gmail handles email from Azure Communication Services.
2. Make sure that the Content-Type header in the email is set correctly when sent through Azure Communication Services. S/MIME-signed emails typically use specific MIME types that need to be properly recognized by the email client.
3. Make sure your Gmail account and other email clients are configured to handle S/MIME signatures correctly.
4. Double-check the configuration settings for Azure Communication Services to make sure there are no restrictions or specific settings that might affect how email is sent.
5. Continue to wait for the technical support from Azure Communication Services tag.

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