Compromised vendor email accounts often lead to business email compromise (BEC) attacks where attackers send malicious or unexpected requests appearing from trusted sources. Training users to be alert to unexpected requests even if they appear to come from familiar addresses is critical in preventing such attacks.
Refraining from clicking images (A) is less effective than being vigilant about suspicious content and requests. Deleting emails from unknown providers (B) is not practical, as some legitimate emails come from unknown senders. Requiring invoices as attachments (C) can increase risk by encouraging users to open potentially malicious attachments.
This user awareness tactic is emphasized in the Security Program Management and Security Awareness training in SY0-701#6:Chapter 16 CompTIA Security+ Study Guide#.