iRedMail will configure Dovecot (POP3/IMAP server) and Roundcube webmail to use the first domain you added during iRedMail installation, so you can login as users without domain part in this domain, because Dovecot and Roundcube will append the domain name for you automatically.
Of course you can change the default domain name:
*) For Dovecot, please open /etc/dovecot/dovecot.conf and update "auth_default_realm =". Reloading or restarting Dovecot service is required.
*) For Roundcube webmail, please open /usr/share/apache2/roundcubemail/config/main.inc.php, update "$rcmail_config['username_domain'] =". Reloading or restarting Apache web service is required.