First, verify that the attachment meets the known file types and size limits supported.

1. Check the Exchange Online admin center to verify supported file types and size limits.
2. Check that the attachment is in the supported file types and size limits.
3. If not, suggest the user convert the attachment to a supported file type.
4. Verify that the user has adequate permissions to access the attachment.
5. Check if the user account is being blocked from sending or receiving the attachment.
6. Test sending and receiving the attachment from an Exchange Online mailbox.
7. If issues persist, contact IT support to troubleshoot further.