Links to apps being flagged by Outlook as potentially malicious

Hi all,

We have a couple of Streamlit apps deployed to Fly. Recently we’ve discovered that emails containing links to these apps, even if they’re in an attached PDF, are “quarantined” by Microsoft Outlook and never delivered to the recipient’s inbox (regardless of whether the links are to fly.io or fly.dev, and regardless of whether they have a Gmail or an Exchange email account). We’ve tried deploying one of these apps to both fly.io and streamlit.io with no difference in code, and only the former link is flagged as a problem by Outlook while the latter comes through fine, so it can’t really be anything to do with the contents of the apps themselves.

Does anyone know whether this is a known issue, and if there is anything to be done - by anyone other than Microsoft - to address it?

(Apparently users of other email clients had the same problem receiving our messages due to their ad blockers or firewalls raising concerns, but I have fewer details on those cases so am focusing on Outlook for now. I am aware that some ad blockers tend to block Segment which we do use for analytics in one of the apps, but surely that’s not reason enough for what we’re seeing here.)

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