The 'Messages can be spoofed' warning in Outlook arises from a multitude of factors, encompassing both technical configurations and organizational policies. These include email security tools (e.g., Proofpoint), incorrect SPF/DKIM/DMARC records, internal security policies, shared hosting environments, or even incorrect system time. The warning can also be a false positive, remediable by whitelisting. Furthermore, the use of free email service providers for 'from' addresses or sender addresses similar to internal contacts may trigger the warning. Internally, stricter security measures by the receiving organization may also cause this warning, irrespective of external authentication.