Hi Audrey,
Welcome to the Atlassian Community!
I've seen a few reports with a similar problem you're describing, but since attaching an email to a card isn't a native feature, we don't fully support it. Thus, I'm afraid we're not able to troubleshoot this problem.
Our guess is that Outlook has a metadata file that is being generated when you drag and drop an email.
To resolve this, I'd suggest you use email-to-board feature instead. It would allow you to create cards from email within a particular board. Here's the article explaining how to set it up: https://support.atlassian.com/trello/docs/creating-cards-by-email/
I hope this helps!
Alina | The Trello Team
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