Where is mail traffic logged?

Nextcloud version: 15
Operating system and version: CENTOS 7
Apache or nginx version: Apache 2.4.6
PHP version: eg, 7.2

The issue you are facing:
No facing an issue… a question.
I setup the IMAP and SMTP with STARTTLS protocol (ports 143 amd 587).
I sent email that arrived and back that was properly recveived. But there are no messages in /var/log/maillog anymore.
Where is receiving and sending of emails logged?

Thanks for your herlp.