MailWizz Email Queue Processing Errors — Diagnosing Failed Campaign Sends

MailWizz campaign send failures appear as campaigns stuck in 'Processing' status, incomplete send counts, or error messages in the campaign logs. Each error type has a distinct cause and resolution path.

SMTP Connection Errors

# Error in MailWizz logs:
# Failed to connect to relay: Connection refused
# Failed to connect to relay: Connection timed out

# Diagnose:
nc -zv smtp-relay-host 2525  # Test port connectivity

# If refused: PowerMTA not running or wrong port configured
# If timeout: Firewall blocking connection between MailWizz and relay

SMTP Authentication Errors

# Error: 535 5.7.8 Authentication credentials invalid
# Fix: Verify username/password match PowerMTA smtp-auth config

# Test credentials manually:
telnet smtp-relay-host 587
EHLO test.com
AUTH LOGIN
[base64-encoded username]
[base64-encoded password]

Template Rendering Errors

# PHP error log shows: Undefined variable in template
# Or: Memory exhausted during template processing

# Fix for memory:
# In php.ini: memory_limit = 512M

# Fix for undefined variables:
# Check template for personalization tags referencing
# custom fields that don't exist in the list

Database Deadlock Errors

# MySQL error log shows: Deadlock found when trying to get lock
# Or MailWizz shows intermittent campaign pausing

# Diagnose:
SHOW ENGINE INNODB STATUS\G
# Look for DEADLOCK section

# Fix: Usually caused by too many parallel workers
# Reduce campaigns.send.parallel or increase innodb timeouts
# innodb_lock_wait_timeout = 50

Campaign Paused by Error Threshold

MailWizz automatically pauses a campaign after a configurable number of consecutive delivery errors. Check Backend → Settings → Campaigns → Max errors before pause. If a campaign is paused due to SMTP errors, fix the underlying SMTP issue before resuming.

Troubleshooting Common Issues

Production MailWizz deployments encounter predictable issues at predictable stages. Understanding the diagnostic workflow for the most common problems in this configuration area saves time and prevents the escalating complexity that comes from applying fixes to a misdiagnosed problem. The diagnostic approach is always the same: identify the symptom precisely (not just "it's not working"), isolate the layer where the failure occurs (MailWizz application, delivery server connection, DNS, ISP rejection), and fix at the correct layer.

Systematic Diagnosis Approach

Check MailWizz logs first (available in Backend → Misc → Application Logs), then check the delivery server SMTP logs, then check the PowerMTA accounting log. Most issues surface in one of these three places. A problem that does not appear in any of these logs is almost always a configuration issue — the system is not attempting what you expect it to attempt.

# MailWizz diagnostic log locations:
# Application logs: Backend → Misc → Application Logs
# Delivery logs: Backend → Campaigns → [Campaign] → Delivery Logs
# Bounce logs: Backend → Bounce Servers → [Server] → Logs

# Server-side logs:
# MailWizz application: /path/to/mailwizz/apps/common/runtime/application.log
# PowerMTA delivery: /var/log/pmta/pmta.log
# PowerMTA accounting: /var/log/pmta/accounting.csv

Performance Optimization for Production Scale

MailWizz performance at scale depends on three infrastructure layers: the web application server (PHP/nginx or Apache), the database (MySQL — query optimization is critical at high subscriber counts), and the delivery infrastructure (PowerMTA connection pool sizing). Performance problems in any of these layers manifest as slow campaign sends, delayed processing, or timeouts that appear unrelated to the specific configuration area being managed.

The most common performance constraint in production MailWizz environments is MySQL query efficiency. As subscriber lists grow beyond 500,000 records, unoptimized database queries for segmentation, bounce processing configuration, and campaign statistics become significant bottlenecks. Ensure that subscriber tables have appropriate indexes on email, status, date_added, and any custom field columns used for segmentation.

# MySQL optimization for large MailWizz installations
# Check slow query log:
SHOW VARIABLES LIKE 'slow_query_log%';
SET GLOBAL slow_query_log = 'ON';
SET GLOBAL long_query_time = 1;  # Log queries over 1 second

# Key indexes to verify exist:
SHOW INDEX FROM mailwizz_lists_subscribers;
# Should have indexes on: email, status, date_added, list_id

# Add missing index if needed:
ALTER TABLE mailwizz_lists_subscribers 
  ADD INDEX idx_email_status (email, status);
  
# Campaign sends table — index on campaign_id + subscriber_id:
ALTER TABLE mailwizz_campaigns_tracking_opens
  ADD INDEX idx_campaign_sub (campaign_id, subscriber_id);

Security Considerations

MailWizz installations handling production sending volumes are valuable targets. Key security practices: use HTTPS for all MailWizz access (including tracking and unsubscribe links), restrict Backend access to authorized IP ranges via web server configuration, rotate API keys periodically and revoke unused keys, maintain regular database backups (automated, offsite), and ensure PHP and MailWizz are kept current with security patches.

The tracking domain (used for open and click tracking) requires special attention: it must have a valid SSL certificate (Let's Encrypt is acceptable), and its DNS records must point exclusively to your MailWizz server. A compromised tracking domain can redirect recipients to malicious sites or reveal subscriber click data to third parties.

Campaign Analytics Integration

Track this MailWizz configuration area through two complementary metric layers: MailWizz campaign statistics (open rate, click rate, bounce rate, unsubscribe rate) and PowerMTA accounting log data (ISP-specific deferral rate, bounce classification, queue depth). Gaps between the two layers reveal delivery problems invisible to MailWizz statistics alone — high MailWizz "sent" counts with elevated PowerMTA deferral rates indicate a queue buildup that campaign dashboards don't surface.

Review campaign metrics against your own historical baselines rather than industry benchmarks. Your list composition, acquisition source, and engagement history define what normal looks like for your environment. Use rolling 7-day and 30-day averages to distinguish trend changes from campaign-specific variance.

Implementation Checklist

Before deploying this configuration to production MailWizz, verify: delivery server connection test passes in Backend → Servers → Delivery Servers, cron jobs are running on the correct schedule, bounce server mailbox is accessible and IMAP credentials are valid, tracking domain has valid SSL and loads within 500ms, and PHP memory limit is set to at least 256MB.

After deploying, send a test campaign to a controlled list of seed addresses across Gmail, Outlook, and Yahoo. Verify Authentication-Results headers show dkim=pass and spf=pass in the received messages. Check that open and click tracking are registering correctly in MailWizz statistics. Confirm bounce processing is updating subscriber status within 15 minutes of a test bounce event.

For managed MailWizz environments operated by Cloud Server for Email, these verification steps are performed automatically after any configuration change. The managed service includes continuous monitoring of delivery server health, cron job execution, and tracking domain availability. Contact infrastructure@cloudserverforemail.com for information about managed MailWizz hosting.

Queue Error Diagnosis and Recovery

The most common queue error in high-volume MailWizz is database connection exhaustion — too many parallel workers competing for database connections during simultaneous campaign processing and tracking writes. Symptoms: campaigns stuck in 'processing', missing open/click events in statistics. Fix: reduce parallel workers, increase MySQL max_connections, enable Redis caching for frequent database reads.

Queue Depth Monitoring

Monitor campaign queue depth by querying the MailWizz campaigns table for campaigns in 'processing' state with a start time more than 2 hours ago. This catches stuck campaigns before operators notice reduced delivery rates. Alert when any campaign has been in 'processing' for more than 3 hours without measurable progress.