Hi KDEPA,
From my understanding, some users of your organization can receive the CSV file and the others are not. And this CSV file could be received in a third-party mailbox.
If I'm right, we suggest you try the steps below to narrow down the root cause of this issue:
1. Check if configure any Inbox rules or archive with these affected user.
2. Trying add another Office 365 account to see whether you'll encounter the same issue.
If not,since file is not list at block list,as I clarified in the previous reply, please use a third-party mailbox to send this csv file to your Office 365 mailbox, then check if you can receive it in your Office 365 mailbox.
Thanks,
Edmond Peng