One of your users has been sent an email and it has been blocked by the Anti-Malware service.
Email blocked by Anti-Malware - Exploit/SpearPhish
Spear Phishing definition
This is an email-spoofing attack that targets specific organization or individual users seeking to gain access to sensitive information by requesting information such as login IDs and passwords. The email will typically appear to come from someone you know by spoofing the sender, as you may be less vigilant and give them the information they ask for. e.g. email coming from Human Resources department or Technical Support department.