SquirrelMail is lightweight but older; it should be deployed with strict hardening and limited plugin surface.
Main file after setup script: config/config.php
Example key settings:
$domain = 'example.com';
$imapServerAddress = 'mail.example.com';
$imapPort = 993;
$smtpServerAddress = 'mail.example.com';
$smtpPort = 587;
$useSendmail = false;
Back up:
config/ and plugin customizationsRecovery test:
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