When setting up a spam trap or mail sinkhole, you'll often need your SMTP server to accept all incoming emails regardless of the recipient address. This configuration is particularly useful for:

• Collecting spam samples for analysis
• Creating honeypot mail servers
• Testing mail routing configurations
• Handling legacy domains where you don't want to lose any emails

The key configuration happens in main.cf. Here's the minimal setup required:

# Enable virtual aliases
virtual_alias_maps = hash:/etc/postfix/virtual

# Accept mail for all domains
mydestination = $myhostname, localhost.$mydomain, localhost
local_recipient_maps =

Create or modify /etc/postfix/virtual with these contents:

@yourdomain.com   catchall
@.   catchall

Then compile the map and reload Postfix:

postmap /etc/postfix/virtual
postfix reload

For more control over the catch-all behavior, consider these additional parameters:

# For better spam handling
smtpd_recipient_restrictions = 
   permit_mynetworks,
   reject_unauth_destination,
   check_policy_service unix:private/policy,
   permit

# Rate limiting to prevent abuse
anvil_rate_time_unit = 60s
smtpd_client_connection_rate_limit = 100

Verify your setup works by sending test emails:

telnet localhost 25
HELO test.com
MAIL FROM: <test@example.com>
RCPT TO: <nonexistent@yourdomain.com>
DATA
Subject: Test
This should be delivered to catchall
.
QUIT

Then check your mail logs:

tail -f /var/log/mail.log

When running a catch-all server:

• Monitor disk space usage (spammers may send large attachments)
• Consider using separate partitions for mail storage
• Implement rate limiting to prevent abuse
• Regularly review the collected emails to detect any legitimate traffic

If emails aren't being caught:

1. Verify Postfix is running: systemctl status postfix
2. Check configuration syntax: postfix check
3. Review mail logs: journalctl -u postfix
4. Test SMTP locally: swaks --to nonexistent@yourdomain.com

---

Sometimes you need an SMTP server that'll accept all incoming emails regardless of recipient address or domain - what we call a "spam sinkhole". This is particularly useful for:

• Capturing misdirected emails during migrations
• Analyzing spam patterns
• Creating honeypot mail servers
• Testing email routing configurations

Here's the core Postfix configuration to make it accept any incoming email:

# main.cf additions
mydestination = $myhostname, localhost.$mydomain, localhost
local_recipient_maps =
virtual_alias_maps = regexp:/etc/postfix/virtual_regexp

Create /etc/postfix/virtual_regexp with:

/.*/ catchall

This regex pattern matches any email address and routes it to the local user 'catchall'.

Ensure the destination mailbox exists:

sudo useradd -m -s /bin/false catchall
sudo mkdir -p /var/mail/catchall
sudo chown catchall:catchall /var/mail/catchall

For more control, you can modify the regex pattern:

# Route specific domains differently
/.*@example\.com$/ example_catchall
/.*@test\.domain$/ test_catchall
/.*/ global_catchall

After reloading Postfix (sudo systemctl reload postfix), test with:

echo "Test email" | mail -s "Test Subject" nonexistent@yourdomain.com

Check if it arrives in the catchall mailbox:

sudo tail -f /var/mail/catchall

When handling large volumes:

• Set default_process_limit = 100 in main.cf
• Adjust qmgr_message_active_limit based on server specs
• Consider using virtual_mailbox_domains for domain-specific routing

Be aware that open relays attract abuse:

# In main.cf to prevent becoming an open relay
smtpd_relay_restrictions = permit_mynetworks, reject_unauth_destination