Using Advanced Threat Protection in Office 365 Email, an added security feature of Safe Attachments is enabled for all users. With Safe Attachments, Email messages are still sent and delivered immediately; however, any file attachments will be delayed around 30 seconds while fully scanned for malicious items. Once the scan is completed, the file attachment is available in the Email message as normal. Microsoft provides the following additional information regarding Safe Attachments as an added level of Email security:
With Safe Attachments, you can prevent malicious attachments from impacting your messaging environment, even if their signatures are not known. All suspicious content goes through a real-time behavioral malware analysis that uses machine learning techniques to evaluate the content for suspicious activity. Unsafe attachments are sandboxed in a detonation chamber before being sent to recipients. The advantage is a malware free and cleaner inbox with better zero-day attack protection.