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Email Processing Settings

Configure how Mailborder handles email flow, delivery, and queue management.

Accessing Email Processing Settings

Navigate to System Settings → Email Processing in the web interface.

Or via command line: 

sudo mb-config show email

Relay Host Configuration

The relay host is where Mailborder delivers clean email (your internal mail server).

Relay Host

Setting: Hostname or IP address of your mail server

Examples: - mail.example.com (recommended - uses DNS) - 192.168.1.10 (IP address) - [mail.example.com] (force no MX lookup)

Via Web Interface: System Settings → Email Processing → Relay Host

Via Command Line: 

sudo mb-config set relay.host mail.example.com

Relay Port

Setting: SMTP port on relay host

Common Values: - 25 - Standard SMTP (most common) - 587 - Submission port - 465 - SMTPS (deprecated but some servers use it) - Custom port if your server uses non-standard

Default: 25

sudo mb-config set relay.port 25

Relay Authentication

If your mail server requires authentication:

Enable Authentication: 

sudo mb-config set relay.auth.enabled true
sudo mb-config set relay.auth.username mailborder
sudo mb-config set relay.auth.password 'SecurePassword123!'

Via Web Interface: System Settings → Email Processing → Relay Authentication - Check "Require Authentication" - Enter username - Enter password

Security Note: Password stored encrypted in database.

Relay TLS/SSL

Encrypt connection to relay host.

Options: - None - No encryption (only for trusted internal networks) - STARTTLS - Opportunistic encryption (recommended) - TLS - Force TLS connection - SSL - Deprecated, avoid

Command Line: 

sudo mb-config set relay.tls starttls

Certificate Verification: 

# Verify relay host certificate (recommended for production)
sudo mb-config set relay.tls.verify true

# Skip verification (for self-signed certs in testing)
sudo mb-config set relay.tls.verify false

Test Relay Connection

After configuring relay host:

Via Web Interface: System Settings → Email Processing → Test Connection

Via Command Line: 

sudo mb-test-relay

Expected output: 

Testing relay connection...
[OK] DNS resolution: mail.example.com → 192.168.1.10
[OK] TCP connection to 192.168.1.10:25
[OK] SMTP greeting received
[OK] EHLO accepted
[OK] TLS negotiation successful
[OK] Authentication successful
[OK] Test message accepted

Relay host is properly configured!

Accepted Domains

Configure which domains Mailborder accepts email for.

Domain Types

Local Domain - Email delivered to relay host - Most common type - Example: example.com, company.net

Relay Domain - Email forwarded without modification - Used for routing to different servers - Example: Hosted service domains

Virtual Alias Domain - Domain aliases (alternate domain names) - All email forwarded to another domain - Example: company.com → company.net

Add Domain

Via Web Interface: 1. Navigate to System Settings → Domains 2. Click "Add Domain" 3. Enter domain name: example.com 4. Select type: Local 5. Click Save

Via Command Line: 

# Add local domain
sudo mb-domain add example.com --type local

# Add relay domain
sudo mb-domain add partner.com --type relay --destination mail.partner.com

# Add virtual alias
sudo mb-domain add company.co --type virtual --destination company.com

List Domains

sudo mb-domain list

Output: 

Domain              Type      Status    Mailboxes
example.com         local     active    150
example.net         local     active    45
oldname.com         virtual   active    → example.com
partner.com         relay     active    → mail.partner.com

Remove Domain

sudo mb-domain remove oldname.com

Reject Unknown Domains

Reject email for domains not in your list.

Recommended: Enabled (prevents open relay)

sudo mb-config set email.reject_unknown_domains true

When disabled, Mailborder accepts email for any domain (relay server mode).

Accepted Recipients

Control which email addresses are accepted.

Recipient Validation

LDAP/AD Integration - Query Active Directory or LDAP for valid recipients - Reject email for non-existent users at SMTP level - Saves bandwidth and processing

Configure LDAP validation: 

sudo mb-config set recipients.ldap.enabled true
sudo mb-config set recipients.ldap.server ldap.example.com
sudo mb-config set recipients.ldap.base_dn "dc=example,dc=com"
sudo mb-config set recipients.ldap.filter "(mail=%s)"

Database Validation - Maintain list of valid recipients in Mailborder database - Manual or API management - Good for smaller deployments

Relay Host Validation - Forward RCPT TO to relay host - Accept if relay accepts - Slower (adds latency) but always accurate

sudo mb-config set recipients.validation relay

Recipient Whitelist

Always accept email for specific addresses regardless of validation:

sudo mb-recipient-whitelist add admin@example.com
sudo mb-recipient-whitelist add support@example.com

Recipient Blacklist

Always reject email for specific addresses:

sudo mb-recipient-blacklist add spam-trap@example.com
sudo mb-recipient-blacklist add old-employee@example.com

Trusted Networks

IP addresses or networks that skip certain checks.

Add Trusted Network

Via Web Interface: System Settings → Email Processing → Trusted Networks

Via Command Line: 

# Single IP
sudo mb-trusted-network add 192.168.1.50

# CIDR range
sudo mb-trusted-network add 192.168.1.0/24

# Entire private network
sudo mb-trusted-network add 10.0.0.0/8

Trusted Network Benefits

Email from trusted networks: - Skip greylisting - Skip some RBL checks - Reduced spam scoring - Faster processing

Use Cases: - Your internal mail servers - Branch office networks - Partner organizations - Internal applications sending email

Trusted Network Limitations

Trusted networks are NOT: - Immune to virus scanning (still scanned) - Immune to content filtering - Immune to recipient validation

Important: Only trust networks you control. Trusting external IPs weakens security.

Email Size Limits

Maximum Message Size

Total size including headers, body, attachments.

Default: 50 MB (52428800 bytes)

# Set to 25 MB
sudo mb-config set email.max_message_size 26214400

# Set to 100 MB
sudo mb-config set email.max_message_size 104857600

Recommended Values: - Small business: 10-25 MB - Standard: 25-50 MB - Large files: 50-100 MB - Enterprise: 100 MB+

Considerations: - Larger = more memory per scan - Larger = slower scanning - Larger = more storage - Must be ≤ relay host limit

Maximum Attachment Size

Per-attachment limit.

Default: 25 MB

sudo mb-config set email.max_attachment_size 26214400

Can be smaller than max message size (multiple small attachments allowed).

Size Limit Actions

When limit exceeded:

  1. Reject (recommended)
  2. Return 552 error at SMTP level
  3. Sender receives bounce
  4. No storage used
sudo mb-config set email.oversize_action reject
  1. Quarantine
  2. Accept but hold for review
  3. Admin can release if legitimate
  4. Uses storage
sudo mb-config set email.oversize_action quarantine
  1. Strip Attachments
  2. Deliver email, remove attachments
  3. Notify recipient of removal
  4. Rare use case

Queue Management

Queue Locations

Mailborder uses multiple queues:

/var/spool/mailborder/
├── incoming/       # New messages
├── active/         # Currently processing
├── deferred/       # Temporary failures (retry queue)
├── hold/           # Admin hold
└── corrupt/        # Damaged messages

Queue Limits

Maximum Queue Size

Total messages allowed in queue.

Default: 10,000

sudo mb-config set queue.max_size 10000

When limit reached: - Postfix returns 421 (try again later) - Sender's server retries automatically - Prevents memory exhaustion

Recommended Values: - Small: 5,000 - Medium: 10,000 - Large: 25,000 - Enterprise: 50,000+

Minimum Free Messages

Reserve queue space for priority email.

Default: 1,000 (10% of max)

sudo mb-config set queue.reserve 1000

Priority email (from trusted networks) can use reserved space.

Queue Retention

Maximum Queue Age

How long messages stay in queue before giving up.

Default: 5 days (432000 seconds)

# 5 days
sudo mb-config set queue.max_age 432000

# 24 hours (aggressive)
sudo mb-config set queue.max_age 86400

# 7 days (patient)
sudo mb-config set queue.max_age 604800

After max age: - Bounce sent to sender - Message removed from queue - Logged as expired

Retry Schedule

For deferred messages (temporary failures).

Default Schedule: - 1st retry: 5 minutes - 2nd retry: 15 minutes - 3rd retry: 1 hour - 4th retry: 4 hours - 5th retry: 12 hours - After 5 days: Give up

Customize: 

sudo mb-config set queue.retry_schedule "5m,15m,1h,4h,12h,24h"

Queue Management Commands

View queue: 

sudo mailq

Count messages: 

sudo mailq | grep -c "^[A-F0-9]"

Flush queue (retry all deferred): 

sudo postqueue -f

Delete specific message: 

sudo postsuper -d MESSAGE_ID

Delete all deferred: 

sudo postsuper -d ALL deferred

Hold message (prevent delivery): 

sudo postsuper -h MESSAGE_ID

Release held message: 

sudo postsuper -H MESSAGE_ID

Connection Limits

Concurrent Connections

SMTP Inbound

Maximum simultaneous connections from Internet.

Default: 100

sudo mb-config set smtp.max_connections 100

Per Source IP

Maximum connections from single IP.

Default: 10

sudo mb-config set smtp.max_per_source 10

Prevents single source from monopolizing resources.

Connection Rate

Maximum new connections per second.

Default: 20

sudo mb-config set smtp.connection_rate 20

Processing Concurrency

Concurrent Scans

Parallel email scanning processes.

Default: Auto (2× CPU cores)

# Manual override
sudo mb-config set processing.concurrent_scans 16

More concurrent = higher throughput but more resource usage.

Timeout Settings

SMTP Timeouts

# How long to wait for client data
sudo mb-config set smtp.timeout.data 300

# How long for client connection
sudo mb-config set smtp.timeout.connect 60

# How long for HELO/EHLO
sudo mb-config set smtp.timeout.helo 60

Scanning Timeout

Maximum time for email scan.

Default: 300 seconds (5 minutes)

sudo mb-config set scanning.timeout 300

If exceeded, email is deferred (retry later).

Bounce Settings

Bounce Address

Return address for bounces.

Default: MAILER-DAEMON@yourhostname

sudo mb-config set bounce.sender "mailborder@example.com"

Bounce Notification

Notify Sender

Send bounce notification to sender.

Default: Enabled

sudo mb-config set bounce.notify_sender true

Notify Postmaster

Copy bounces to postmaster.

Default: Disabled

sudo mb-config set bounce.notify_postmaster false
sudo mb-config set bounce.postmaster_address "postmaster@example.com"

Email Headers

Header Modifications

Add Headers

Mailborder adds these headers by default:

X-Mailborder-Server: mailborder.example.com
X-Mailborder-Version: 6.0.0
X-Spam-Score: 2.5
X-Spam-Status: No, score=2.5
X-Virus-Scanned: ClamAV

Customize Header Prefix: 

sudo mb-config set headers.prefix "X-Mailborder"

Add Custom Headers: 

sudo mb-header add "X-Company-ID" "ABC123"
sudo mb-header add "X-Mail-Gateway" "Mailborder"

Remove Headers:

Strip headers before delivery:

# Remove all X-Originating-IP headers
sudo mb-header remove "X-Originating-IP"

# Remove Received headers (privacy - careful!)
sudo mb-header remove "Received"

Header Privacy

Hide Internal IPs

Remove internal IP addresses from headers.

sudo mb-config set headers.hide_internal_ips true

Replaces internal IPs with [HIDDEN].

Hide Received Headers

Remove Received headers showing internal mail flow.

Warning: May break SPF/DKIM. Only use if required for privacy.

sudo mb-config set headers.hide_received false

Advanced Settings

Null Sender

Accept email with null sender (MAIL FROM:<>).

Used for bounces and notifications.

Default: Enabled

sudo mb-config set email.allow_null_sender true

Address Extensions

Support "plus addressing" (user+tag@example.com).

Default: Enabled

sudo mb-config set email.allow_address_extensions true

Examples: - john+shopping@example.com - support+ticket123@example.com

All delivered to base address (john@example.com).

Case Sensitivity

Email addresses are case-insensitive per RFC.

Default: Lowercase all addresses

sudo mb-config set email.lowercase_addresses true

VERBS SMTP Command

Allow SMTP VRFY (verify user exists).

Default: Disabled (security - information disclosure)

sudo mb-config set smtp.allow_vrfy false

ETRN SMTP Command

Allow SMTP ETRN (trigger queue flush for domain).

Default: Disabled

sudo mb-config set smtp.allow_etrn false

Configuration Examples

Example 1: Small Office

# Single internal mail server
sudo mb-config set relay.host mail.office.local
sudo mb-config set relay.port 25
sudo mb-config set relay.auth.enabled false

# Accept one domain
sudo mb-domain add company.com --type local

# Trust internal network
sudo mb-trusted-network add 192.168.1.0/24

# Conservative limits
sudo mb-config set email.max_message_size 26214400  # 25 MB
sudo mb-config set queue.max_size 5000

Example 2: Office 365 Backend

# Office 365 as relay
sudo mb-config set relay.host company-com.mail.protection.outlook.com
sudo mb-config set relay.port 25
sudo mb-config set relay.tls starttls
sudo mb-config set relay.tls.verify true

# Multiple domains
sudo mb-domain add company.com --type local
sudo mb-domain add company.net --type local

# No authentication needed (IP-based)
sudo mb-config set relay.auth.enabled false

# Standard limits
sudo mb-config set email.max_message_size 52428800  # 50 MB

Example 3: Multi-Server Environment

# Load-balanced relay hosts
sudo mb-config set relay.host mail-lb.company.com
sudo mb-config set relay.port 25

# High volume settings
sudo mb-config set queue.max_size 50000
sudo mb-config set processing.concurrent_scans 32
sudo mb-config set smtp.max_connections 500

# Multiple trusted networks
sudo mb-trusted-network add 10.0.0.0/8
sudo mb-trusted-network add 172.16.0.0/12

Applying Changes

After configuration changes:

# Reload configuration
sudo systemctl reload mb-virtuoso postfix

# Verify configuration
sudo mb-config --verify

# Test relay connection
sudo mb-test-relay

# Monitor logs
sudo tail -f /var/log/mailborder/virtuoso.log

Troubleshooting

Email Not Relaying

Check relay configuration: 

sudo mb-config get relay.host
sudo postconf relayhost

Test connectivity: 

telnet mail.example.com 25

Check logs: 

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

Queue Growing

Check queue size: 

sudo mailq | wc -l

Check deferred reasons: 

sudo mailq | grep -A 2 "^[A-F0-9]"

Common causes: - Relay host down or slow - Authentication failure - Network issues - Relay host rejecting

Flush queue: 

sudo postqueue -f

Rejected Recipients

Check domain acceptance: 

sudo mb-domain list

Test recipient validation: 

sudo mb-test-recipient user@example.com

Check logs: 

sudo grep "reject" /var/log/mail.log | tail -20

Next Steps