454 4.7.0 Temporary authentication failure: generic failure

SYSTEM INFORMATION
OS type and version Ubuntu Linux 20.04.5
Webmin version 2.013
Virtualmin version 7.5
Usermin version 1.861

Hello, We are using virtualmin to manage server which is being used to send email campaigns using mautic, email client is roundcube.

Everything was working fine, deliverability was good, but after we have reached a huge number of emails, both sent and received things broke down.

When sending from roundcube, we can no longer send email and it shows error:
SMTP Error (454): Authentication failed
Everything was working fine, before we have reached scale.

Exact error log is:
454 4.7.0 Temporary authentication failure: generic failure

The error message indicates that the SMTP server (Postfix) at domain.com is experiencing a “Temporary authentication failure” with the SASL authentication mechanism. This issue is occurring during the SMTP authentication process with the username test1@domain.com.

So problem lies with saslauthd.

Any ideas how to fix this?

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