# Mautic 5 Slow Email Sending – 25k Contacts via Custom SMTP Taking Too Long
**URL:** https://forum.mautic.org/t/mautic-5-slow-email-sending-25k-contacts-via-custom-smtp-taking-too-long/37360
**Category:** Product Support
**Tags:** community, volunteer, discussion, development, email
**Created:** [February 12, 2026, 12:23pm UTC](https://forum.mautic.org/t/mautic-5-slow-email-sending-25k-contacts-via-custom-smtp-taking-too-long/37360 "2026-02-12T12:23:10Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@gandhi](https://forum.mautic.org/u/gandhi)
#### Post date: [February 12, 2026, 12:23pm UTC](https://forum.mautic.org/t/mautic-5-slow-email-sending-25k-contacts-via-custom-smtp-taking-too-long/37360/1 "2026-02-12T12:23:10Z")
**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?
---
### Author:  [@marcus42](https://forum.mautic.org/u/marcus42)
#### Post date: [February 14, 2026, 3:52pm UTC](https://forum.mautic.org/t/mautic-5-slow-email-sending-25k-contacts-via-custom-smtp-taking-too-long/37360/2 "2026-02-14T15:52:49Z")
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).
---
### Author:  [@gandhi](https://forum.mautic.org/u/gandhi)
#### Post date: [February 18, 2026, 1:27pm UTC](https://forum.mautic.org/t/mautic-5-slow-email-sending-25k-contacts-via-custom-smtp-taking-too-long/37360/3 "2026-02-18T13:27:03Z")
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.