Sendmail

Configuring Sendmail for Laravel

Overview

Sendmail is a classic Mail Transfer Agent (MTA) with a long history in Unix-based systems. While modern alternatives like Postfix and Exim dominate new deployments, Sendmail remains valuable for legacy systems, deep customization, and specific integration scenarios.

This guide is not a recommendation to use Sendmail everywhere, but rather a comprehensive reference for developers who need to configure it—whether working with legacy systems, complex requirements, or custom mail handling.

When to Use Sendmail

✅ Good use cases:

  • Integrating with existing Sendmail infrastructure
  • Advanced customization and local mail handling
  • Legacy systems requiring Sendmail compatibility
  • Direct SMTP control without external services

❌ Not recommended for:

  • New deployments (use Postfix or Exim instead)
  • Simple hosting setups (use managed SMTP services)
  • Beginners (Postfix is much simpler)

System Requirements

Before installing Sendmail, ensure:

Component Requirement Notes
OS Linux, BSD, macOS Unix-based systems only
RAM 256 MB minimum 512 MB+ recommended
Disk 1 GB minimum For queue and logs
m4 macro processor Required sudo apt-get install m4
PHP 5.2+ For proc_open() function

Key Limitation

Important: Laravel's Sendmail transport uses proc_open(), a PHP function that must be enabled. If disabled in php.ini (common in shared hosting), you cannot use Sendmail with Laravel.

Installation

Install Sendmail

Ubuntu/Debian:

sudo apt-get update
sudo apt-get install sendmail sendmail-bin m4
sudo systemctl enable sendmail
sudo systemctl start sendmail

CentOS/RHEL:

sudo yum install sendmail sendmail-cf m4
sudo systemctl enable sendmail
sudo systemctl start sendmail

Verify installation:

sendmail -v

Understanding Sendmail Configuration

.mc vs .cf Files

Sendmail's configuration system is unique and often confusing:

File Type Purpose Editable?
.mc file Human-readable configuration ✅ Yes (edit this)
.cf file Compiled by m4; used by Sendmail ❌ No (don't edit directly)

The workflow:

.mc file → m4 processor → .cf file → Sendmail reads .cf

The m4 Macro Processor

Sendmail uses m4 to compile .mc files into .cf files. Key concepts:

  • Processes as stream — Order of directives matters
  • dnl command — "Delete through newline"; used for comments
  • Macro expansion — Happens even in comments; can cause surprises

Convert .mc to .cf:

m4 /etc/mail/cf/m4/cf.m4 sendmail.mc > sendmail.cf

Basic .mc File Structure

A proper Sendmail .mc file follows this order:

# Version control
VERSIONID(`$Id: example.mc,v 1.0 2024/01/01 $')dnl

# Operating system (REQUIRED)
OSTYPE(`linux')dnl

# Domain configuration (optional but recommended)
DOMAIN(`generic')dnl

# Feature definitions (must come before MAILER)
FEATURE(`use_cw_file')dnl
FEATURE(`mailertable')dnl
FEATURE(`virtusertable')dnl

# Macro definitions (depends on FEATURE)
define(`SMART_HOST', `smtp.example.com')dnl
define(`MAIL_HUB', `mail.example.com')dnl

# Local customizations
LOCAL_CONFIG
# Custom rules here

# Mailer definitions (MUST BE LAST)
MAILER(`local')dnl
MAILER(`smtp')dnl

Critical rule: Place MAILER() directives last. Features that depend on macros must come before those macros are used.

Essential Configuration Directives

OSTYPE (Required)

Specifies the operating system. This is mandatory and controls paths, flags, and defaults.

OSTYPE(`linux')dnl

Common values:

  • linux — Linux systems
  • bsd44 — BSD variants
  • solaris — Solaris/SunOS
  • hpux — HP-UX
  • aix — IBM AIX

Without OSTYPE, the build will fail.

DOMAIN (Optional but Recommended)

Sets domain-specific configurations like local names and masquerading:

DOMAIN(`generic')dnl

Or custom domain (create /etc/mail/domain/example.com.m4):

DOMAIN(`example.com')dnl

MAILER (Required)

Defines which mailers Sendmail uses. local is included by default, but you must explicitly include smtp for outgoing mail:

MAILER(`local')dnl
MAILER(`smtp')dnl

Available mailers:

  • local — Local delivery (included automatically)
  • smtp — SMTP variants (smtp, esmtp, smtp8, dsmtp, relay)
  • uucp — UUCP delivery (legacy)
  • procmail — Pipe to procmail
  • cyrus — Cyrus mailbox format
  • fax — HylaFAX delivery

FEATURE Macros

Enable optional functionality:

FEATURE(`use_cw_file')dnl           # Read /etc/mail/local-host-names
FEATURE(`use_ct_file')dnl           # Read trusted users
FEATURE(`mailertable')dnl           # Enable mailer table routing
FEATURE(`virtusertable')dnl         # Virtual domain hosting
FEATURE(`always_add_domain')dnl     # Add domain to local addresses
FEATURE(`generics_entire_domain')dnl # Apply generics to subdomains

Configuration for Laravel

Step 1: Create/Edit .mc File

Create /etc/mail/sendmail.mc:

VERSIONID(`$Id: sendmail.mc,v 1.0 2024/01/01 $')dnl
OSTYPE(`linux')dnl
DOMAIN(`generic')dnl

FEATURE(`use_cw_file')dnl
FEATURE(`mailertable')dnl
FEATURE(`always_add_domain')dnl

define(`confLOG_LEVEL', `9')dnl
define(`confDOMAIN_NAME', `example.com')dnl

LOCAL_CONFIG
# Local rules can go here

MAILER(`local')dnl
MAILER(`smtp')dnl

Step 2: Compile to .cf File

cd /etc/mail
m4 m4/cf.m4 sendmail.mc > sendmail.cf
sudo chown root:wheel sendmail.cf
sudo chmod 644 sendmail.cf

Step 3: Restart Sendmail

sudo systemctl restart sendmail

Step 4: Configure Laravel

Update .env:

MAIL_MAILER=sendmail
MAIL_SENDMAIL="/usr/sbin/sendmail -bs"

Update config/mail.php:

return [
    'default' => env('MAIL_MAILER', 'sendmail'),
    'mailers' => [
        'sendmail' => [
            'transport' => 'sendmail',
            'path' => env('MAIL_SENDMAIL', '/usr/sbin/sendmail -bs'),
        ],
    ],
];

Clear Laravel's configuration cache:

php artisan config:cache

Address Rewriting and Masquerading

Masquerading (Hiding Internal Addresses)

Rewrite outgoing mail addresses to appear to come from your domain:

MASQUERADE_AS(`example.com')dnl
FEATURE(`masquerade_envelope')dnl  # Masquerade envelope too

Example:

  • Internal user: john@mail.example.com
  • Appears as: john@example.com

Masquerade Exceptions

Exclude certain hosts from masquerading:

MASQUERADE_EXCEPTION(`localhost')dnl
MASQUERADE_EXCEPTION(`mail.example.com')dnl

Generic Maps (Address Rewriting)

Rewrite local addresses to different domains:

Create /etc/mail/genericstable:

root                admin@example.com
www-data            web@example.com
@localhost          user@example.com

Add to .mc file:

FEATURE(`generics_entire_domain')dnl
define(`GENERICS_TABLE_DOMAIN_NAMES', `example.com')dnl

Build the map:

makemap hash /etc/mail/genericstable < /etc/mail/genericstable
sudo systemctl restart sendmail

Virtual Domains

Virtual User Table

Host multiple domains without creating system users:

Create /etc/mail/virtusertable:

support@example.com         localuser
admin@example.net           admin@example.com
error@another.org           /dev/null
info@              error    "550 No such user here"

Add to .mc file:

FEATURE(`virtusertable')dnl

Build the map:

makemap hash /etc/mail/virtusertable < /etc/mail/virtusertable
sudo systemctl restart sendmail

Map entries explained:

  • address@domain username — Deliver to local user
  • address@domain user@other.com — Forward to external address
  • address@domain /dev/null — Silently discard
  • @domain error "message" — Reject with error message

Relaying and Routing

Smart Host (Central Relay)

Send all outgoing mail through a relay server:

define(`SMART_HOST', `smtp.isp.com')dnl

Mail Hub (Local Host Relay)

Send mail for local hostnames to a central hub:

define(`MAIL_HUB', `mailhub.example.com')dnl

Local Relay (Unqualified Names)

Forward unqualified addresses (just username) to a relay:

define(`LOCAL_RELAY', `relay.example.com')dnl

Mailer Tables

Route mail for specific domains to different mail servers:

Create /etc/mail/mailertable:

.example.com        smtp:mail.example.com
partner.com         smtp:[partner-mail.com]:2525
legacy.org          uucp-new:legacyhost!
.                   smtp:[smtp.isp.com]

Add to .mc file:

FEATURE(`mailertable')dnl

Build the map:

makemap hash /etc/mail/mailertable < /etc/mail/mailertable
sudo systemctl restart sendmail

Syntax:

  • .domain — Matches domain and subdomains
  • domain — Exact match only
  • . — Default/catch-all

Security Configuration

TLS/STARTTLS

Enable encrypted SMTP connections:

define(`confCACERT_PATH', `/etc/ssl/certs')dnl
define(`confCACERT', `/etc/ssl/certs/ca-certificates.crt')dnl
define(`confSERVER_CERT', `/etc/letsencrypt/live/mail.example.com/fullchain.pem')dnl
define(`confSERVER_KEY', `/etc/letsencrypt/live/mail.example.com/privkey.pem')dnl

SMTP Authentication

Allow authenticated relay (combat spam):

define(`confAUTH_MECHANISMS', `EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
define(`confAUTH_OPTIONS', `A p')dnl

Anti-Spam Rules

Restrict relaying to authorized users:

FEATURE(`relay_hosts_only')dnl
FEATURE(`loose_relay_check')dnl

LOCAL_CONFIG
R$*                $: $&{client_name}

Custom Rulesets

LOCAL_RULE_3 (Canonicalization)

Customize address canonicalization:

LOCAL_RULE_3
# Map old hosts to new domains
R$* @ oldhost $*    $1 @ newhost.example.com $2

LOCAL_RULESETS (Custom Rules)

Add completely custom rulesets:

LOCAL_RULESETS

Scheck_from_ok
R$*                $: ok

LOCAL_CONFIG (Custom Declarations)

Define custom maps and variables:

LOCAL_CONFIG
KscheduleMap hash -o /etc/mail/schedulemap
Kldap ldap -1 -v sendmailMTAmap -b dc=example,dc=com

Using Lookup Tables

Berkeley DB Maps

Default map type for most Sendmail configurations:

makemap hash /etc/mail/mapname < /etc/mail/mapname.txt

Change Default Map Type

define(`DATABASE_MAP_TYPE', `dbm')dnl

LDAP Integration

Query LDAP directory for lookups:

LOCAL_CONFIG
Kldap_aliases ldap -1 -v mail -b ou=aliases,dc=example,dc=com
Kldap_users ldap -1 -v mailLocalAddress -b ou=people,dc=example,dc=com

Troubleshooting Sendmail

Check Sendmail Status

sudo systemctl status sendmail
netstat -tulnp | grep sendmail

View Sendmail Logs

# Real-time monitoring
sudo tail -f /var/log/mail.log

# Search for errors
grep "ERROR" /var/log/mail.log

# Find rejected mail
grep "reject" /var/log/mail.log

# Trace a specific message
grep "MESSAGE_ID" /var/log/mail.log

Increase Log Level

Add to .mc file:

define(`confLOG_LEVEL', `9')dnl

Then recompile and restart.

Test Email Sending

Manually test Sendmail:

echo "Subject: Test Email

This is a test." | sendmail -v recipient@example.com

Verify Configuration

Check syntax before restarting:

sendmail -C /etc/mail/sendmail.cf -bi

Common Issues and Solutions

Issue: "sendmail not found"

sudo apt-get install sendmail sendmail-bin

Issue: "Emails not sending from Laravel"

  • Check proc_open() is not disabled in php.ini
  • Verify /usr/sbin/sendmail exists and is executable
  • Check web server user can execute sendmail
  • Review logs: tail -f /var/log/mail.log

Issue: "Permission denied on sendmail"

sudo chmod 4755 /usr/sbin/sendmail
sudo chown root:smmsp /usr/sbin/sendmail

Issue: ".cf file not found" errors

  • Ensure you compiled .mc to .cf: m4 /etc/mail/cf/m4/cf.m4 sendmail.mc > sendmail.cf
  • Restart Sendmail after moving .cf file
  • Check file permissions: sudo chmod 644 /etc/mail/sendmail.cf

Issue: "Relay access denied"

  • Check SMART_HOST configuration
  • Verify MASQUERADE_AS is set if needed
  • Review access database: makemap -q hash /etc/mail/access

Working with the Mail Queue

View Queue

# List all queued messages
mailq

# Count messages
mailq | wc -l

# Show just message IDs
mailq | grep "^[A-F0-9]"

Flush Queue (Force Delivery)

# Retry all messages
sendmail -q

# Force immediate processing
sendmail -q -v

# Process specific queue run
sendmail -qI <QUEUE_ID>

Remove Stuck Messages

# Remove specific message
sudo rm /var/spool/postfix/deferred/<QUEUE_ID>

# Clear entire queue (dangerous!)
sudo rm /var/spool/postfix/deferred/*

System Mail Forwarding

Redirect system mail (cron, services) to real person:

Create/Edit /etc/aliases:

root: admin@example.com, admin@backup-domain.com
www-data: web-admin@example.com

Build the alias database:

sudo newaliases

This ensures important system messages reach administrators, not root's inbox.

Performance Tuning

Connection Settings

define(`confTO_CONNECT', `10s')dnl
define(`confTO_INITIAL', `10s')dnl
define(`confTO_QUEUERETURN', `5d')dnl
define(`confMAX_HEADERS_LENGTH', `32768')dnl

Concurrency Limits

define(`confMAX_DAEMON_CHILDREN', `20')dnl
define(`confMAX_QUEUE_CHILDREN', `20')dnl
define(`confQUEUE_LA', `8')dnl
define(`confREFUSE_LA', `12')dnl

Process Limits

define(`confMAX_MESSAGE_SIZE', `100000000')dnl
define(`confNO_RCPT_ACTION', `add-to-undisclosed')dnl

Best Practices

Configuration Management

✅ Do:

  • Keep .mc files in version control
  • Backup /etc/mail/sendmail.cf before changes
  • Test configuration in development first
  • Document all customizations
  • Use m4 to compile, never edit .cf directly

❌ Don't:

  • Edit .cf files directly
  • Forget to rebuild when modifying .mc
  • Use Sendmail on shared hosting (if proc_open is disabled)
  • Keep passwords in .mc files unencrypted

Security Checklist

  • [ ] Run Sendmail as unprivileged user (sendmail)
  • [ ] Restrict file permissions: chmod 600 for sensitive files
  • [ ] Enable TLS for SMTP connections
  • [ ] Implement SASL authentication for relaying
  • [ ] Configure SPF and DKIM records
  • [ ] Monitor logs regularly
  • [ ] Keep Sendmail updated
  • [ ] Test open relay prevention

Monitoring

Regular checks to perform:

# Queue health
mailq | grep "^[A-F0-9]" | wc -l

# Recent errors
grep "ERROR" /var/log/mail.log | tail -20

# Authentication attempts
grep "AUTH" /var/log/mail.log | tail -10

# Rejected messages
grep "reject" /var/log/mail.log | tail -10

Advanced Topics

LDAP-Based Routing

Implement LDAP-based email routing with:

FEATURE(`ldap_routing')dnl
LOCAL_CONFIG
Kldaproute ldap -1 -v mailLocalAddress

Mail Filters (Milters)

Integrate SpamAssassin or other mail filters:

define(`confINPUT_MAIL_FILTERS', `spamcheck')dnl
define(`confMILTER_DEFAULT_ACTION', `accept')dnl

Queue Groups

Organize queue by destination:

QUEUE_GROUP(`fast', `P=/var/spool/mqueue-fast, F=')dnl
QUEUE_GROUP(`slow', `P=/var/spool/mqueue-slow, F=')dnl

Migration Guide

From Sendmail to Postfix

If moving away from Sendmail:

  1. Back up all Sendmail configs and maps
  2. Set up Postfix with equivalent configuration
  3. Test mail flow on development system
  4. Plan cutover during low-traffic window
  5. Monitor logs for issues
  6. Keep Sendmail running as backup initially
  7. Redirect SMTP port to Postfix when confident

Quick Reference

Common .mc Directives

VERSIONID(`...')           # Version control
OSTYPE(`linux')            # Operating system
DOMAIN(`generic')          # Domain configuration
FEATURE(`use_cw_file')     # Read /etc/mail/local-host-names
FEATURE(`mailertable')     # Enable mailer routing
FEATURE(`virtusertable')   # Virtual domains
MAILER(`local')            # Local delivery
MAILER(`smtp')             # SMTP delivery
define(`SMART_HOST', `...')        # Relay server
define(`MASQUERADE_AS', `...')     # Masquerade domain

Common Files

  • /etc/mail/sendmail.mc — Source configuration
  • /etc/mail/sendmail.cf — Compiled configuration
  • /etc/mail/local-host-names — Accepted hostnames
  • /etc/mail/genericstable — Address rewriting
  • /etc/mail/virtusertable — Virtual domains
  • /etc/mail/mailertable — Routing rules
  • /etc/aliases — Local aliases
  • /var/log/mail.log — Sendmail logs
  • /var/spool/mqueue/ — Mail queue

Common Commands

m4 /etc/mail/cf/m4/cf.m4 sendmail.mc > sendmail.cf    # Compile
makemap hash /etc/mail/mapname < /etc/mail/mapname.txt # Build maps
newaliases                                               # Rebuild aliases
sendmail -C /etc/mail/sendmail.cf -bi                   # Verify
mailq                                                     # View queue
sendmail -q                                               # Flush queue
sudo systemctl restart sendmail                          # Restart