A mail server that stops responding on SMTP, IMAP, or POP3 blocks delivery for an entire organization, often before anyone notices a bounced message. Nagios XI checks mail server and protocol availability, confirms messages complete a full send-and-receive round trip, and tracks Microsoft Exchange and Microsoft 365 health, then alerts the team responsible once a check moves to Warning or Critical.

Email monitoring checks whether a mail server is reachable, whether its Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), and Post Office Protocol (POP3) services respond correctly, and whether a message completes the trip from submission to an inbox within an expected time. It covers on-premises mail servers, Microsoft Exchange, and cloud mailbox platforms such as Microsoft 365.

A failed protocol or a full mail queue rarely announces itself before messages start bouncing.

• Catch outages before users do: Protocol and server checks move to Warning or Critical as soon as a mail service stops responding, not after a backlog of failed sends.
• Confirm messages actually arrive: Send-and-receive tests verify a message reaches the inbox within your threshold, not just that a port is open.
• Spot capacity problems early: Queue length and server resource checks flag a backlog or a full disk before it blocks delivery.
• Route alerts to the right team: Contact groups send mail server alerts to the people responsible for messaging infrastructure, not general server or network staff.
• Cover hybrid and cloud mail from one place: The same dashboard tracks on-premises mail servers and Microsoft Exchange alongside Microsoft 365 mailboxes.

Email Monitoring Capabilities in Nagios XI

Mail Server and Protocol Availability

The Mail Server Wizard adds your mail server as a Host and lets you select which protocols to monitor: SMTP, IMAP, and POP3. Each check confirms the protocol responds and moves to Warning or Critical when it doesn’t.

• Protocol response checks: Confirm SMTP, IMAP, and POP3 are accepting connections on their configured ports.
• Host availability: Confirm the mail server itself is reachable, independent of which protocol is failing.

Documentation: How To Use The Mail Server Wizard In Nagios XI

End-to-End Send and Receive Testing

The Email Delivery Wizard monitors a specific email account’s ability to send and receive messages. It pings the mail server, then sends a test message over SMTP and confirms its arrival over IMAP.

• Full round trip: SMTP delivers the test message, and IMAP confirms it arrived in the destination mailbox, then deletes it.
• Dedicated test account: Point the check at a mailbox set aside for testing, with its own SMTP and IMAP credentials, rather than a live user’s account.
• Works against hosted mail: Point the wizard at internal mail servers or hosted providers such as Gmail and Microsoft 365 by entering their SMTP and IMAP endpoints.

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

Microsoft Exchange Server Monitoring

The Exchange Server Wizard connects through the Nagios Cross-Platform Agent (NCPA) to on-premises Exchange servers from version 5.5 through 2019 and monitors the services behind mail flow and Outlook access. Track mailbox databases, transport queues, and client access in more depth with Microsoft Exchange monitoring.

• Protocol and uptime checks: An Internet Control Message Protocol (ICMP) ping check watches latency and uptime, alongside checks that verify SMTP, IMAP, and POP service availability.
• Blacklist status: An RBL (real-time blackhole list) check reports whether your mail server’s IP address appears on public DNS-based blacklists.
• Outlook Web Access: An Outlook Web Access (OWA) HTTPS check confirms OWA is reachable over HTTPS.
• Core and web services: Checks confirm that the Windows services Exchange depends on, and its web services, are running.
• Exchange-specific metrics: Checks vary by version but can include messages queued for delivery, messages pending routing, and remote queue length.

Documentation: How To Monitor MS Exchange Servers With Nagios XI | Installing NCPA

Microsoft 365 and Exchange Online Monitoring

The Microsoft 365 Wizard connects to your tenant through an Azure app registration and monitors mailbox, SharePoint, and licensing metrics without installing anything on Microsoft’s servers. Track usage and licensing across your tenant in more depth with Microsoft 365 monitoring.

• Mailbox metrics: Mailbox activity, mailbox usage, and mailbox counts, tracked by user.
• Licensing and activations: Office 365 user and product activations, and license activation counts.
• SharePoint: Site activity, storage, active files, and active users.
• Group and service usage: Office 365 user product and service usage, and group activity and files.

Documentation: How To Use The Microsoft 365 Wizard In Nagios XI

Mail Server Performance and Resource Metrics

Add the mail server as a Host in a Linux or Windows monitoring wizard alongside your protocol checks to track the resources that cause slowdowns and bounces before they affect delivery.

• CPU, memory, and disk: Resource checks on the mail server flag the exhaustion that precedes a queue backup.
• Queue depth for Exchange: The Exchange Wizard tracks messages queued for delivery, messages pending routing, and remote queue length, so a backlog is visible before users notice delays.

Watch the volume holding your mail queue and message store with disk space monitoring before it fills and blocks delivery.

Custom Mail Port and Protocol Checks

The TCP/UDP Port Wizard checks custom or non-standard mail ports, including submission or relay ports that aren’t covered by the standard protocol checks.

• Common server ports: Select from a list of common server ports, or add a custom port with its own name.
• Send and expect strings: Send a string to the port and match it against an expected response, confirming the service is not just open but behaving correctly.

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

Email Alert Thresholds and Notifications

Each wizard sets Warning and Critical thresholds per check, so a slow SMTP response and a dead mail server trigger different levels of urgency.

• Per-check thresholds: Set numeric Warning and Critical values for response time, queue length, and resource checks during the wizard, or adjust them later in Core Config Manager.
• Contact routing: Route mail server alerts to the contact group responsible for messaging infrastructure, separate from general server or network alerts.

Mail Security and Access Checks

Confirm the certificate behind STARTTLS and encrypted IMAP and POP3 hasn’t lapsed with SSL certificate monitoring.

• Blacklist status: The Exchange Wizard’s RBL check reports whether your server’s IP address is listed on public DNS-based blacklists, which can silently block outbound delivery to other providers.
• Dedicated test credentials: Use an account and mailbox set aside for the Email Delivery Wizard’s SMTP and IMAP 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. Reserve Admin rights, which include access to Configuration Wizards, Users, and Security Credentials, for the people who manage mail monitoring.

Documentation: How To Understand User Rights In Nagios XI

Fewer surprise outages. Protocol and server checks catch a stopped SMTP service or a down mail server before a backlog of undelivered messages reaches your help desk.

Faster root-cause identification. Separate checks for server, protocol, queue, and resource status mean an alert points at what actually failed, instead of just reporting that email is down.

One view across on-premises and cloud mail. Exchange servers, generic SMTP, IMAP, and POP3 mail servers, and Microsoft 365 tenants report into the same dashboard and the same notification rules.

Confidence that messages are actually delivered. Send-and-receive testing verifies a full round trip, which catches failures a simple port check would miss.

• Network path: The Nagios XI server needs outbound access to your mail server on the ports you intend to monitor. SMTP commonly uses ports 25, 465, and 587; IMAP uses ports 143 and 993; and POP3 uses ports 110 and 995, depending on whether the connection is encrypted.
• Dedicated test mailbox: The Email Delivery Wizard needs its own mailbox with SMTP credentials to send and IMAP credentials to inspect and delete the test message, such as <TEST_MAILBOX_USER> and <TEST_MAILBOX_PASSWORD>.
• NCPA for Exchange: The Exchange Server Wizard connects through NCPA, so the agent must be installed on the Exchange server before you run the wizard.
• Azure app registration for Microsoft 365: The Microsoft 365 Wizard needs an Azure app registration with a client secret and the Group.Read.All, Reports.Read.All, User.Read.All, and Sites.Read.All Microsoft Graph API permissions, plus Global Administrator consent to grant them.

Documentation: Understanding Nagios XI Ports and Protocols

1. Add your mail server and protocol checks. Navigate to Configure > Configuration Wizards, search for “Mail Server,” and run the Mail Server Wizard to add the Host and choose which of SMTP, IMAP, and POP3 to monitor.
2. Confirm end-to-end delivery. Run the Email Delivery Wizard against a dedicated test mailbox, entering its SMTP and IMAP details, to verify a message can be sent and received within your expected timeframe.
3. Add Exchange-specific checks. If you run Microsoft Exchange, install NCPA on the Exchange server, then run the Exchange Server Wizard for mailbox, queue, and Outlook Web Access checks.
4. Add Microsoft 365 checks. Register an Azure app with the required Graph API permissions, then run the Microsoft 365 Wizard to monitor mailbox usage, licensing, and SharePoint.
5. Set thresholds and notifications. Adjust Warning and Critical thresholds for response time, queue length, and resource checks, 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 | How To Monitor MS Exchange Servers With Nagios XI | How To Use The Microsoft 365 Wizard In Nagios XI

Nagios XI covers the mail server and mailbox layer: whether the server and its protocols respond, whether a test message completes a full send-and-receive round trip, and whether the host has the resources to keep processing mail. It does the same for Microsoft Exchange and Microsoft 365.

Searching and retaining the mail server’s own logs, such as Postfix or Exchange message tracking logs, across many servers over time is a separate capability, covered by Nagios Log Server, not Nagios XI.

Email & Messaging

• Microsoft Exchange Monitoring
• Microsoft 365 Monitoring
• SMTP Monitoring
• IMAP Monitoring
• POP Monitoring
• Postfix Monitoring
• Dovecot Monitoring
• Sendmail Monitoring

Infrastructure and Security

• DNS Monitoring
• Disk Monitoring
• SSL Certificate Monitoring

Add your mail server, Exchange environment, or Microsoft 365 tenant to Nagios XI and get protocol, delivery, and resource checks reporting into the same dashboard you already use for the rest of your infrastructure.