When configuring Monit to send email alerts through Gmail's SMTP server, several authentication and connection issues can occur. The error message "error receiving data from the mailserver 'smtp.gmail.com' -- Resource temporarily unavailable" typically indicates one of these underlying problems:

// Typical error pattern seen in monit logs
Sendmail: error receiving data from the mailserver 'smtp.gmail.com'
Alert handler failed, retry scheduled for next cycle

Here's a verified configuration that works with modern Gmail requirements:

set mailserver smtp.gmail.com port 587
    username "your.email@gmail.com" password "your-app-password"
    using tlsv1
    with timeout 30 seconds
    using hostname "smtp.gmail.com"

set mail-format {
    from: your.email@gmail.com
    subject: $SERVICE $EVENT at $DATE
    message: Monit $ACTION $SERVICE at $DATE on $HOST: $DESCRIPTION.
}

set alert recipient@domain.com

• Use an App Password instead of your regular Gmail password if you have 2FA enabled
• Ensure TLSv1 or higher is specified (TLSv1.2 recommended)
• Verify your firewall isn't blocking outbound port 587
• Check Google Account security settings for "Less secure app access" if not using App Password

Test your SMTP configuration independently using this command:

echo "Test email body" | mailx -s "Test Subject" -S smtp="smtp.gmail.com:587" \
-S smtp-use-starttls -S smtp-auth=login -S smtp-auth-user="your.email@gmail.com" \
-S smtp-auth-password="yourpassword" recipient@example.com

If this test fails, the issue lies with your SMTP setup rather than Monit specifically.

If email continues to be problematic, consider these alternatives:

# Slack webhook example
check file alerttest with path /.nonexistent
    if does not exist then exec "/usr/bin/curl -X POST -H 'Content-type: application/json' \
    --data '{\"text\":\"Alert: file missing\"}' https://hooks.slack.com/services/your/webhook"

When configuring Monit with Gmail's SMTP server, developers often encounter these specific error patterns:

Sendmail: error receiving data from the mailserver 'smtp.gmail.com' -- Resource temporarily unavailable
Alert handler failed, retry scheduled for next cycle

Here's the full working configuration that addresses multiple potential failure points:

set mailserver smtp.gmail.com port 587
    username "your.email@gmail.com" password "app-specific-password"
    using tlsv1
    with timeout 30 seconds
    using hostname "yourdomain.com"  # Important for some Gmail configurations

set alert your.email@domain.com

1. App Password Requirement: Since May 2022, Google requires app passwords for third-party apps:

• Enable 2FA on your Google account
• Generate an app password specifically for Monit

2. Firewall Considerations: Ensure outgoing connections to port 587 aren't blocked:

sudo ufw allow out 587/tcp  # For Ubuntu systems

To isolate the problem, test SMTP connectivity directly:

openssl s_client -connect smtp.gmail.com:587 -starttls smtp -crlf

Expected response should include:

250 SMTPUTF8

If standard configuration fails, try these variations:

# Option 1: Explicit TLS declaration
set mailserver smtp.gmail.com port 587
    username "user@gmail.com" password "pass"
    using tls
    with timeout 30 seconds

# Option 2: Different TLS version
using tlsv1_2

Check Monit's mail queue status:

monit status

Look for entries like:

Mail queue status: 1 message(s) pending

After configuration changes:

1. Reload Monit: monit reload
2. Force immediate check: monit monitor alerttest
3. Check logs: tail -f /var/log/monit.log