Emails from Outlook clients are being blocked

Hi community - so we have a mail server setup (mailcow) and we also use OnlyOffice portal for the users for mail, docs, etc. All works ok except that 6/10 times when an email is forwarded from Outlook we get this message.

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Any idea why?

Hello @mrsc
Are you using Workspace Cloud or self-hosted Workspace installed on your own server? What is the exact issue scenario of this pop-up message? If it’s possible, please record a video file of your test.