A Sendmail server that stops accepting Simple Mail Transfer Protocol (SMTP) connections or runs out of queue space blocks outbound and inbound mail for everyone behind it. Nagios XI checks Sendmail’s SMTP availability, confirms a message can complete a full send-and-receive round trip, and tracks the host resources that keep the mail queue moving.

Sendmail is an open-source mail transfer agent (MTA) that some organizations still run to route and deliver mail over SMTP. Sendmail monitoring checks whether the process is running, whether it responds correctly on SMTP, and whether a test message completes a full send-and-receive round trip.

Sendmail deployments are often long-running systems with less day-to-day oversight than newer mail platforms, which makes a silent failure easy to miss.

• Catch outages before users do: A stopped Sendmail process or a failed SMTP check triggers an alert instead of a growing pile of bounced messages.
• Confirm mail actually moves: Send-and-receive tests verify a message reaches its destination, not just that the port is open.
• Spot a growing queue early: Resource and disk checks on the mail spool flag a backlog before it fills the disk and stops delivery entirely.
• Route alerts to the right team: Contact groups send Sendmail alerts to whoever owns the mail server, separate from general infrastructure alerts.

Server and Protocol Availability

Sendmail speaks standard SMTP, so Nagios XI monitors it the same way it monitors any mail server: the Mail Server Wizard adds the Host and checks whether SMTP responds on its configured port.

• Protocol response: Confirms SMTP is accepting connections and responding correctly.
• Host availability: Confirms the underlying host is reachable, independent of the SMTP check.
• Non-standard ports: If Sendmail listens on a custom port, the TCP/UDP Port Wizard checks it directly, including an optional send string and expected response.

Documentation: How To Use The Mail Server Wizard In Nagios XI | How To Use The TCP/UDP Configuration Wizard In Nagios XI

Send and Receive Testing

The Email Delivery Wizard pings the mail server, sends a test message over SMTP, and confirms its arrival over Internet Message Access Protocol (IMAP) against a dedicated test mailbox.

• Full round trip: Verifies a message is sent, delivered, and received within your expected timeframe, not just that the server answers a ping.
• Dedicated test account: Point the check at a mailbox set aside for testing rather than a live user’s account.

Documentation: How To Use the Email Delivery Wizard In Nagios XI

Server Resources and Mail Queue

Add the Sendmail host to a Linux monitoring wizard alongside your protocol checks to track the resources that keep mail moving. A growing queue typically shows up first as disk consumption on the mail spool volume.

• CPU, memory, and disk: Resource checks on the host flag the exhaustion that precedes a queue backup.

Watch the volume holding your mail spool with disk space monitoring before a backlog fills it and stops delivery.

Alert Thresholds and Notifications

Each check carries its own Warning and Critical thresholds, so a slow SMTP response and a dead Sendmail process trigger different levels of urgency. Set thresholds during the wizard, or adjust them later in Core Config Manager, and route alerts to the contact group responsible for mail.

Security and Access

Confirm the certificate behind STARTTLS hasn’t lapsed with SSL certificate monitoring.

• Dedicated test credentials: Use an account set aside for the Email Delivery Wizard’s checks, such as <TEST_MAILBOX_USER> and <TEST_MAILBOX_PASSWORD>, rather than an administrator or live user account.
• Role-based visibility: Nagios XI user accounts default to the restrictive User authorization level, which limits a user to Hosts and Services where they’re listed as a contact.

Fewer surprise outages. Protocol and resource checks catch a stopped Sendmail process or a filling queue before messages start bouncing.

Confidence mail is actually delivered. Send-and-receive testing verifies a full round trip, which a simple port check would miss.

One view alongside the rest of your infrastructure. Sendmail checks report into the same dashboard and notification rules as your other Hosts and Services.

1. Add the host and SMTP check. Navigate to Configure > Configuration Wizards, search for “Mail Server,” and run the Mail Server Wizard against your Sendmail host.
2. Confirm end-to-end delivery. Run the Email Delivery Wizard against a dedicated test mailbox to verify a message can be sent and received.
3. Add resource checks. Install NCPA or NRPE on the Sendmail host, then run the matching Linux wizard for CPU, memory, and disk checks.
4. Set thresholds and notifications. Adjust Warning and Critical thresholds, then confirm alerts route to the contact group responsible for mail.

Documentation: How To Use The Mail Server Wizard In Nagios XI | How To Use the Email Delivery Wizard In Nagios XI

Nagios XI covers the Sendmail host and protocol layer: whether the process and SMTP service respond, whether a test message completes a full round trip, and whether the host has the resources to keep processing mail. For mail server monitoring across other protocols and platforms, see email monitoring.

Searching and retaining Sendmail’s own logs across many servers over time is a separate capability, covered by Nagios Log Server, not Nagios XI.

Email & Messaging

• Email Monitoring
• Postfix Monitoring
• SMTP Monitoring
• IMAP Monitoring
• POP Monitoring
• Dovecot Monitoring

Infrastructure

• Linux Monitoring
• Disk Monitoring
• SSL Certificate Monitoring

Add your Sendmail server to Nagios XI and get protocol, delivery, and resource checks reporting into the same dashboard you already use for the rest of your infrastructure.