fail2ban uses its action configuration to send email notifications through your system's MTA (Mail Transfer Agent). The most common setup uses sendmail command, but it can be configured to work with other MTAs like Postfix or Exim.

First, verify your jail.local contains proper email settings:

# /etc/fail2ban/jail.local
[DEFAULT]
destemail = admin@yourdomain.com
sender = fail2ban@yourdomain.com
mta = sendmail
action = %(action_mwl)s

Create a custom action file to test notifications:

# /etc/fail2ban/action.d/testmail.conf
[Definition]
actionstart = echo "fail2ban started" | mail -s "fail2ban startup notification" 
actionstop = echo "fail2ban stopped" | mail -s "fail2ban shutdown notification" 
actioncheck = 
actionban = 
actionunban = 

You can manually trigger test emails by restarting fail2ban:

sudo systemctl restart fail2ban

Or create a test jail that triggers on demand:

# /etc/fail2ban/jail.d/testmail.conf
[testmail]
enabled = true
filter = 
action = testmail
logpath = /dev/null

If emails aren't arriving:

1. Check system mail logs: journalctl -u postfix (or your MTA)
2. Test MTA independently: echo "test" | mail -s "test" your@email.com
3. Verify fail2ban logs: tail -f /var/log/fail2ban.log

For more control, create a bash script:

#!/bin/bash
# /usr/local/bin/fail2ban-mailer.sh
SUBJECT="fail2ban $1 notification"
BODY="fail2ban service has $1 at $(date)"
echo "$BODY" | mail -s "$SUBJECT" admin@yourdomain.com

Then configure your action:

actionstart = /usr/local/bin/fail2ban-mailer.sh start
actionstop = /usr/local/bin/fail2ban-mailer.sh stop

<h2>Understanding fail2ban Email Configuration</h2>

<p>Before testing email notifications, ensure your fail2ban is properly configured for SMTP. Here's a typical jail.local configuration snippet:</p>

<pre><code>[DEFAULT]

# Email settings

destemail = admin@yourdomain.com

sender = fail2ban@yourdomain.com

mta = sendmail

action = %(action_mwl)s

</code></pre>
<h2>Manual Trigger Testing Method</h2>

<p>The most reliable way to test email functionality is by manually triggering fail2ban events:</p>
<pre><code># Restart fail2ban to trigger startup notification

sudo systemctl restart fail2ban
# Check mail logs for delivery attempts

sudo tail -f /var/log/mail.log

</code></pre>
<h2>Creating a Test Jail for Email Verification</h2>

<p>For more controlled testing, create a dedicated test jail:</p>
<pre><code>[test-email]

enabled = true

filter = %(test-email.filter)s

logpath = /var/log/test-email.log

maxretry = 1

findtime = 60

bantime = 60

action = %(action_mwl)s

</code></pre>
<p>Then create a matching filter:</p>
<pre><code>[INCLUDES]

before = common.conf
[Definition]

failregex = ^.*Test email trigger.*$

ignoreregex =

</code></pre>
<h2>Forcing Email Notification with fail2ban-client</h2>

<p>Use fail2ban-client to directly test email sending:</p>
<pre><code>sudo fail2ban-client set <jailname> actionban "curl -s --user \

'api:YOUR_MAILGUN_API_KEY' \

https://api.mailgun.net/v3/YOUR_DOMAIN/messages \

-F from='fail2ban <fail2ban@yourdomain.com>' \

-F to=admin@yourdomain.com \

-F subject='[fail2ban] TEST: Banned <ip>' \

-F text='Test email body'"

</code></pre>
<h2>Debugging Email Delivery Issues</h2>

<p>If emails aren't received, check these critical points:</p>
<ul>

<li>Verify SMTP credentials in /etc/fail2ban/action.d/sendmail-*.conf</li>

<li>Test SMTP connectivity with telnet or swaks</li>

<li>Inspect fail2ban logs: sudo tail -f /var/log/fail2ban.log</li>

<li>Check mail queue: sudo mailq</li>

</ul>
<pre><code># Example SMTP test using telnet

telnet your.smtp.server 25

EHLO yourdomain.com

MAIL FROM: <fail2ban@yourdomain.com>

RCPT TO: <admin@yourdomain.com>

DATA

Subject: Test email

This is a test email.

.

QUIT

</code></pre>