Mautic 5 Slow Email Sending – 25k Contacts via Custom SMTP Taking Too Long

Product Support
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1

Your software
My Mautic version is:5.0.4
My PHP version is:8.2
My Database type and version is:MySQL

SMTP : Custom SMTP (KumoMTA)

Your problem
My problem is:
I configured a custom SMTP (KumoMTA) with Mautic and launched a campaign to send emails to around 25k contacts.

However, the campaign is extremely slow:

  • After 14–15 hours, only ~12,000 emails were sent

  • Campaign is still running

  • Throughput is roughly 800–900 emails/hour

Based on debugging and logs, it looks like:

Mautic is sending emails one-by-one over SMTP, opening/closing connections repeatedly instead of batching or parallel sending.

Because of this, sending speed is very slow compared to other tools (e.g., I tried with same SMTP with other tool it sends much faster).

Email configuration

  • Transport: SMTP

  • Host: KumoMTA

  • Port: 25

  • Encryption: None

  • Sending method: Immediate (not queued)

Is there a recommended configuration for high-volume SMTP sending?

2

You may not send multiple emails simultaneously through a single transaction.

In other words, if you’re sending 50 emails, it needs to make 50 different HELO, regardless of your client (this is how the SMTP work).

Probably a misconception about PHP (Mautic is a simple PHP app. Nothing fancy, really.)
PHP is a single threaded interpreter. It doesn’t support parallel tasks (never did, never will). It’s always one after the other.

Your “slow sending” experience is most likely due to a slow server (it’s not about the quantity but the quality of the hardware).

3

In our solution, we configured the queue system and used the following command to improve email sending performance:

sudo -u www-data bash -c ‘cd /var/www/html/mautic && for i in {1..5}; do php bin/console messenger:consume -vv email --limit=200 --time-limit=3600 --memory-limit=512M --no-reset > var/logs/worker_$i.log 2>&1 & done’

This command starts 5 parallel workers, each consuming email messages from the queue.

As a result, emails are processed concurrently instead of sequentially, which significantly increases the overall sending speed.