# SMTP - authentication failure - (one more time...)

**URL:** <https://forum.yunohost.org/t/smtp-authentication-failure-one-more-time/41344>\
**Category:** Support\
**Tags:** english\
**Created:** [January 18, 2026, 10:17pm UTC](https://forum.yunohost.org/t/smtp-authentication-failure-one-more-time/41344 "2026-01-18T22:17:13Z")\
**Posts on this page:** 1\
**Showing post:** 13

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**Author:** ![marc](https://forum.yunohost.org/user_avatar/forum.yunohost.org/marc/32/10973_2.png) [@marc](https://forum.yunohost.org/u/marc)\
**Post date:** [February 22, 2026, 10:50pm UTC](https://forum.yunohost.org/t/smtp-authentication-failure-one-more-time/41344/13 "2026-02-22T22:50:52Z")

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Hi @otm33 @djez @jarod5001

I got the solution! At least, after many hours of trials and debugging!

very simple, but how difficult to resolve:

in Yunohost dovecot configuration (for example: in `/etc/dovecot/yunohost.d/post-ext.conf` ) just add :

`auth_cache_size = 10M`

### For those who will encounter the same kind of problems:

It is possible to add debug messages to dovecot and postfix using:

> **[Postfix Debugging Howto](https://www.postfix.org/DEBUG_README.html)**

and

> **[Debugging | Dovecot CE](https://doc.dovecot.org/main/developers/debugging.html)**
>
> Dovecot CE Documentation

For my side, i used the debug parameters in dovecot:

`log_debug = (category=auth) OR (category=smtp)`

Then i could analyse the log file with the command:

```auto
sudo tail -f /var/log/mail.log 

```

Here is how SMTP works:

- first your mail client connects to Postfix
- in /etc/postfix/main.cf, we can see that dovecot is used by Postfix to authenticate the user (with `smtpd_sasl_type = dovecot `parameter)
- so Postfix sends a connexion request to Dovecot
- Then dovecot goes through LDAP to authenticate the user

In my case, the time to authenticate was about 14 seconds. Postfix can not wait more than 10 seconds, and this delay can not be tuned ! There is no such parameter in Postfix config file. I tried a lot, but no one works!

But Dovecot proposes [some guidelines](https://doc.dovecot.org/2.3/configuration_manual/performance_tuning/) to tune performance, and especially [for CPU](https://doc.dovecot.org/2.3/configuration_manual/performance_tuning/#:~:text=CPU%20usage%20optimization%C2%B6)

There is the solution !

I hope these explanations will help !

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