For example, I’ll take the installed iRedMail-1.6.8 and change the maximum size of the mail attachment.
First let’s change the Postfix configuration:
nano /etc/postfix/main.cf
message_size_limit = 31457280
mailbox_size_limit = 31457280
/etc/init.d/postfix restart
Then in PHP-FPM:
nano /etc/php/8.1/fpm/php.ini
memory_limit = 256M;
upload_max_filesize = 30M;
post_max_size = 32M;
service php8.1-fpm restart
In Roundcube:
nano /opt/www/roundcubemail/config/config.inc.php
$config['max_message_size'] = '30M';
In the Nginx web server settings:
nano /etc/nginx/conf-enabled/client_max_body_size.conf
client_max_body_size 30m;
service nginx restart
Done, now you can send an email attachment up to the specified size via an email client or the roundcube web interface.