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

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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?