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Clear out junk files and repair common Windows errorsFree Scan →Scan for outdated or missing drivers - takes under a minuteDriver Scan →To investigate suspicious outbound SMTP traffic from a Linux server, preserve the current process and socket state, identify which process and account own each connection, and correlate the activity with network, DNS, application, authentication, and mail logs. Then compare what you find with the server’s intended role and normal traffic. An unfamiliar destination or process is a lead—not proof of compromise.
1. Establish scope and preserve evidence
Before stopping services, killing processes, deleting files, or restarting the host, record the facts needed to interpret the evidence:
- Hostname, Linux distribution and version, timezone, and current time.
- The suspected time window and how it was identified.
- The server’s role and whether it is expected to send mail directly or through a relay.
- The alert, firewall or flow records, relevant DNS information, and available mail-relay records.
When incident conditions allow, capture volatile state first: process lists, listening and established sockets, and open-file information. Preserve relevant host and network logs as well. CISA’s joint investigation guidance and technical guidance identify useful evidence sources, including journald, /var/log, cron, systemd, account data, suspicious temporary files, kernel module listings, and SSH authorized keys.
With appropriate permissions, these commands can help record an initial snapshot:
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date -u
hostnamectl
ps auxfww
ss -tpn
lsof -nP -iSave command output with timestamps and, if feasible, copy it to a trusted location outside the potentially compromised host. The commands are examples, not a distribution-independent forensic procedure; available tools, permissions, and output vary. The lsof manual describes its role in listing open files, including network-related information.
2. Characterize the traffic before judging it
Use firewall, flow, EDR, or authorized packet telemetry to establish what the traffic did and when. Record the source host or process if available, destination address and domain, port and protocol, connection frequency, timing, and bytes or message volume. Determine whether the connections repeat and whether their pattern changed.
Compare those observations with the server’s purpose and historical baseline. For example, a web application that is configured to submit messages through an approved relay may reasonably make outbound SMTP-related connections; an unexpected direct connection to an unfamiliar destination warrants investigation. CISA recommends analyzing frequency and patterns against normal network behavior and notes that outbound data movement can use varied ports and protocols in its activity analysis guidance. Unusual timing, volume, or destination is not conclusive on its own.
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If packet capture is justified and authorized, use an approved collection point and limit the capture to the incident need. Avoid collecting message bodies or credentials unnecessarily. The cited guidance supports collecting network evidence but does not establish a universal Linux capture command or retention period for this scenario.
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1Repair Windows errors before they cause bigger problems2Scan for outdated or missing drivers - takes under a minute3Clear out junk files and repair common Windows errors3. Identify the process and account behind each connection
For each suspicious socket, map the connection to its process ID, executable, command line, parent process, user, start time, and open files. Preserve the socket and process output together so their timestamps and identifiers can be compared.
Check whether the executable’s location and package ownership make sense for the host. Investigate processes running from writable temporary directories, deleted executable paths, unexpected interpreters, unusual service children, or processes that appear at the same time as the outbound connections. Treat these as leads to validate—not automatic proof of malicious activity.
Connect process identity to deployment history, service configuration, and the expected mail path. A legitimate application can send through an approved relay, and an attacker may abuse a valid application or credential; neither a familiar process name nor a particular port establishes that the activity is authorized.
4. Correlate host, mail, DNS, and network records
Review records covering the same time window, allowing for timezone differences and clock skew. Relevant sources may include the system journal, syslog files, authentication logs, application and web-server logs, firewall records, DNS resolver logs, and mail transfer agent (MTA) logs. Look for authentication attempts, application errors followed by outbound connections, new scheduled work, configuration changes, and DNS lookups matching the connection destinations. CISA recommends preserving and correlating host and network logs in its technical approaches guidance.
For an authorized mail service
Compare the sender or envelope identity, recipient domains, relay, timestamps, message or session identifiers, response codes, and traffic volume with expected service behavior. Microsoft’s message-tracking documentation and connector documentation illustrate useful mail-flow pivots, but their procedures and log formats apply to Exchange, not Linux MTAs.
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Linux mail-log locations, formats, and queue-inspection commands depend on the MTA, distribution, and local configuration. Do not assume a universal Postfix or Exim command or log path; use the documentation for the software actually installed.
When credentials or application exposure may be involved
Review which application or account could have submitted the mail, whether its credentials were exposed, and whether related application activity preceded the connections. CISA’s Androxgh0st advisory describes SMTP scanning and abuse of exposed credentials as capabilities. That makes credential and application review relevant, but SMTP-like traffic alone does not identify Androxgh0st or any other malware family.
5. Check for persistence and broader compromise
Look beyond the process making the connection. Review cron entries, systemd services and timers, new or modified accounts, service-account shell assignments, SSH authorized keys, and recent package or executable changes. Check suspicious files in /tmp, /var/tmp, or /dev/shm; inspect kernel module listings and boot or system logs when the evidence and capabilities warrant it. These artifact classes appear in CISA’s joint investigation guidance. Validate unusual findings against approved administration and deployment records before treating them as unauthorized.
6. Decide on containment after initial collection
Once useful initial evidence is secured, choose containment actions through the incident-response process. Depending on scope and business impact, options may include blocking a destination, disabling an account or credential, stopping a process, restricting outbound traffic, isolating the host, or routing mail through a known-good relay. Consider whether a partial action could disrupt operations, alert an active adversary, or impair a broader investigation. CISA advises sequencing mitigation with the goals of understanding scope and achieving full eviction in its technical approaches guidance; it also recommends considering third-party incident-response support when appropriate.
After containment, rotate exposed SMTP and application credentials from a trusted system, review related hosts and accounts, remediate the entry point, validate the host’s mail configuration, and monitor for recurrence. Retain relevant logs and artifacts in the incident record.
Compare explanations against the same evidence
Use consistent checks to distinguish authorized mail, misconfiguration, credential abuse, and possible host compromise:
| Evidence | Questions to ask |
|---|---|
| Process and account | Does the process, executable, and user match an approved application or service? |
| Destination and relay | Does the destination match the approved mail relay or service configuration? |
| Timing and volume | Do connection timing and message or byte volume fit the service’s normal baseline? |
| Related records | Do mail, application, authentication, DNS, and network logs show an expected job or event, or unexplained credential use? |
| Independent host evidence | Are there separately validated signs of persistence, account changes, or unauthorized modifications? |
One suspicious connection rarely settles the question. A stronger conclusion comes from multiple records that agree on the process, account, destination, timing, and surrounding host activity.
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