Unusual hostname for the sending mail server in the email headers

Synopsis

Field	Value
Activation Period	14 Days
Training Period	30 Days
Test Period	N/A (single event)
Deduplication Period	1 Day
Required Data	Microsoft 365 Emails
Detection Modules	Email
Detector Tags	Spoofing
ATT&CK Tactic	Stealth (TA0005)
ATT&CK Technique	Social Engineering: Impersonation (T1684.001)
Severity	Informational

Description

The detected mail server hostname had not been observed in the organization's emails in the past 30 days.

Attacker's Goals

Disguise the email's origin by spoofing the received header to appear as a trusted sender, impersonating a trusted source, aims to mislead recipients into disclosing private data or performing unsafe acts.

Investigative actions

• Review the email's received headers, to trace its path and spot spoofing signs.
• Examine the sender's IP address and domain reputation.
• Closely inspect the email content for malicious links, attachments, or requests for sensitive information.
• Monitor further actions taken, such as file downloads or access to potentially malicious links.