Configuring Sendmail as a Queueing Relay for External MTA with Failover Support

When dealing with multiple applications that send emails through an external mail transfer agent (MTA), service interruptions can cause significant operational issues. The need arises for a local queueing mechanism that can:

Accept emails from internal applications
Queue messages during external MTA outages
Relay messages to the destination MTA when available

The most effective way to implement this is through sendmail's SMART_HOST feature combined with proper queue management. Here's the core configuration strategy:

# In your sendmail.mc file:
define(SMART_HOST', [external.mta.example.com]')dnl
FEATURE(queuegroup')dnl
FEATURE(greet_pause')dnl

1. Basic Relay Setup

# /etc/mail/sendmail.mc configuration:
define(confQUEUE_LA', 5')dnl        # Start queueing when load avg > 5
define(confMIN_QUEUE_AGE', 30m')dnl # Minimum time in queue before retry
define(confTO_QUEUEWARN', 4h')dnl   # Send delay notification after 4h
define(SMART_HOST', smtp:[external.mta.example.com]')dnl

2. Queue Groups for Better Management

QUEUE_GROUP(relayq', P=/var/spool/mqueue/relay, R=2, I=1m')dnl
QUEUE_GROUP(localq', P=/var/spool/mqueue/local')dnl
define(confQUEUE_GROUP', relayq')dnl

3. Network Timeout Adjustments

define(confTO_CONNECT', 1m')dnl
define(confTO_ICONNECT', 1m')dnl
define(confTO_HELO', 5m')dnl
define(confTO_MAIL', 10m')dnl

For your case where emails have arbitrary destination domains but come from a fixed internal domain, we can use mailertable:

# /etc/mail/mailertable
.    relay:[external.mta.example.com]

Then rebuild the mailertable database:

makemap hash /etc/mail/mailertable.db < /etc/mail/mailertable

After making these changes, remember to:

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

# Restart sendmail
service sendmail restart

# Test the configuration
echo "Subject: Test" | sendmail -f support@company.com user@externaldomain.com

Regularly check your queue with:

mailq
sendmail -bp
sendmail -q -v  # Process queue in verbose mode

For true high availability, consider setting up multiple external MTAs:

# In mailertable:
.    relay:[primary.mta.example.com],[backup1.mta.example.com],[backup2.mta.example.com]

Tune these parameters based on your mail volume:

define(confMAX_QUEUE_CHILDREN', 50')dnl
define(confMAX_DAEMON_CHILDREN', 100')dnl
define(confMIN_FREE_BLOCKS', 100')dnl

When dealing with mission-critical email delivery from multiple applications, implementing a local queueing mechanism becomes essential. The common pain point is external MTA outages causing application-level failures. Sendmail can serve as an effective buffer between your applications and the external mail infrastructure.

The most efficient approach involves using both SMART_HOST for outbound routing and queue management parameters:

# In sendmail.mc
define(SMART_HOST', smtp.ext-mta.example.com')dnl
define(confQUEUE_LA', 5')dnl        # Queue when load avg > 5
define(confMIN_QUEUE_AGE', 30m')dnl # Retry interval
define(confTO_QUEUEWARN', 4h')dnl   # Warn after 4hrs in queue
define(confTO_QUEUERETURN', 5d')dnl # Return undeliverable after 5 days

For more granular control over specific domains while maintaining the default relay:

# /etc/mail/mailertable
.example.com       esmtp:[ext-mta.example.com]
backup.com         esmtp:[secondary-mta.example.net]

Configure your queue runner for optimal performance:

# In sendmail.mc
define(confDELIVERY_MODE', background')dnl
define(confMAX_QUEUE_CHILDREN', 16')dnl
define(confREFUSE_LA', 12')dnl      # Reject new connections at LA 12

Essential commands for queue management:

# Force queue processing
/usr/sbin/sendmail -q -v

# Check queue status
mailq

# Test configuration
sendmail -bt << EOF
parse user@example.com
EOF

When relaying through an external MTA, ensure proper authentication:

# In sendmail.mc
define(confAUTH_MECHANISMS', EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')dnl
FEATURE(authinfo', hash /etc/mail/authinfo.db')dnl