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In Plesk Obsidian 18.x , you may encounter webmail.domain.com page returns 500 error
How to Fix
The issue is caused by webmail is using PHP with minimal extension, and some essential PHP extension for webmail application is missing.
The default lsphp comes with LSWS installation is lsphp56 ,so the solution is to replace the it to higher version that installed by Plesk.
rm -f /usr/local/lsphp ln -s /opt/plesk/php/7.3/bin/lsphp /usr/local/bin/lsphp
Replace 7.3 to other Plesk PHP version if PHP 7.3 is not installed on your server.