In this tutorial, you’ll learn how to monitor mail server availability and performance using PRTG Network Monitor.
This step-by-step guide shows how to configure SMTP, IMAP, and POP3 monitoring sensors.
Mail server issues can disrupt business communication without warning.
PRTG helps IT administrators detect email service problems early with real-time alerts.
In this video, we demonstrate how to add a mail server and configure monitoring thresholds correctly.
You’ll also learn best practices for mail server monitoring and alert notifications.
This tutorial is ideal for system administrators and IT professionals.
Follow this guide to ensure your email services remain reliable and available.

LAB: I’m using Mailcow with Docker as my mail server model. The monitoring process is similar to that of custom-built mail servers.

Step 1: Add Device

Enter SSH username/password (Key is recommended)

Step 2: Add Sensors

I. VM INFRASTRUCTURE (MANDATORY)

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1️⃣ Ping

Function
• Check if the mailcow VM is online
Threshold
• ❌ No need to set upper/lower
• DOWN on timeout
📌 Default PRTG is standard
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2️⃣ SSH Load Average v2
Function

• Monitor the total CPU load of the mail server

Channel used
• Load Average (1 min)
Threshold
• ⚠️ Upper WARNING: CPU core × 1.5
• ❌ Upper DOWN: CPU core × 2.5
📌 Example:
• VM 2 core →
o Warning: 3
o Down: 5

3️⃣ SSH Memory Usage v2

Function

• Monitor RAM (mailcow + Docker use a lot of RAM)

Channel used
• Percent Available Memory
Threshold
• ⚠️ Lower WARNING: 20%
• ❌ Lower DOWN: 10%

Note:
Linux does NOT consider cache/buffer as “used”: RAM cache/buffer will be reclaimed as soon as the app needs it.
Summary: VM includes cache/buffer in the used data. To find out exactly, use the command `free -h` in the VM mail server.

4️⃣ SSH Disk Free v2

Functions

• Monitor disk space (mail full disk = mail server down)

Channel used
• Free Space %
Threshold
• ⚠️ Lower WARNING: 15%

• ❌ Lower DOWN: 5%

II. MAIL CORE SERVICES
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5️⃣ SMTP (Port 25/587)

Functions

• Check mail server receives mail from external sources
Threshold
• ❌ No need to set
• DOWN if unable to connect
📌 Important for inbound mail
Create SMTP sensor 587

6️⃣ SMTP Secure (465)

Still using SMTP sensor
Functions

• Check secure mail sending (SSL)

Threshold
• ❌ No need to set
• DOWN if SSL handshake fail
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7️⃣ IMAP Secure (993)

Sensor: IMAP
Function
• Client receives mail (Outlook, Thunderbird, Mobile)

Threshold
• ❌ No setting required
• DOWN = user does not receive mail
📌 Required
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III WEB & ADMINISTRATION
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8️⃣ HTTP Advanced – Mailcow UI

Function

• Check Mailcow administration interface
URL
https://mail.domain.com

Channel used
Loading time: Total page load time (Reference: Nginx/PHP/Mailcow backend)
Threshold
• ⚠️ Upper WARNING: 4000 ms
• ❌ Upper DOWN: 10000 ms

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9️⃣ HTTP Advanced – SOGo Webmail

Function
• Check SOGo webmail for User
URL
https://mail.tsf.id.vn/SOGo

Thresholds
• ⚠️ Upper WARNING: 4000 ms

• ❌ Upper DOWN: 10000 ms

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IV SSL & SECURITY
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🔟 SSL Certificate

Functions

• Check SSL certificate expiration date

Channel used
• Days to Expiration
Thresholds

• ⚠️ Lower WARNING: 15 days

• ❌ Lower DOWN: 7 days

Disable Revoked channel due to using Let’s Encrypt SSL

Note: SSL Let encrypt turn off rekove