Python-generated S/MIME email via Microsoft Graph: Outlook compatibility issues

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1

I’m stuck with a weird problem. I made an S/MIME email using Python and sent it through Microsoft Graph. It works fine in Gmail and on my iPhone, but Outlook is giving me trouble.

Here’s what’s happening in Outlook:

  1. It says there’s something wrong with the certificate
  2. The email turns into a winmail.dat attachment
  3. The S/MIME signature looks messed up, and it fails DKIM and DMARC checks

Any ideas why Outlook is acting up? Could it be something to do with how Outlook handles S/MIME or maybe how the Graph API formats the email?

I’m pretty confused since it works everywhere else. Anyone run into something like this before?

2

hmm, sounds like a tricky one. outlook can be pretty finicky with s/mime sometimes. have you tried checking the encoding of your email headers? maybe outlook is interpreting them differently. also, double-check your certificate chain - outlook might be more strict about validating it. good luck troubleshooting!

3

This issue seems to stem from Outlook’s stringent S/MIME implementation. First, verify your certificate is from a trusted CA and hasn’t expired. The winmail.dat problem might be due to TNEF encoding; try disabling Rich Text Format in your Graph API call.

For the signature issues, ensure you’re using the correct signing algorithm (SHA-256 is recommended) and that your DKIM and DMARC records are properly set up in your DNS. It’s also worth checking if your email headers comply with RFC 5322 standards, as Outlook can be particularly sensitive to non-standard formatting. If the problem persists, consider reaching out to Microsoft support for Graph API-specific guidance.

4

I’ve dealt with similar Outlook headaches before. It’s likely a combination of factors. First, Outlook’s S/MIME implementation is notoriously picky. Make sure your certificate is up-to-date and from a trusted CA. The winmail.dat issue often crops up with TNEF encoding - try tweaking your MIME settings in the Graph API call.

As for the signature problems, double-check your signing algorithm (SHA-256 is usually a safe bet) and verify your DKIM/DMARC setup. Outlook can be really sensitive to non-standard email formatting, so it might be worth reviewing your headers against RFC standards.

If you’re still stuck, you might need to dig into Outlook’s S/MIME logs for more clues. It’s a pain, but sometimes that’s the only way to pinpoint what’s triggering Outlook’s pickiness. Hang in there!