You can do it
To change the "welcome to Horde"
Edit: /etc/psa-horde/horde/registry.php
Find: 'name' => _("Horde"),
Replace with:
'name' => _(preg_replace('/^webmail./', '', $GLOBALS['HTTP_SERVER_VARS']['SERVER_NAME']) ),
OR
by changing this line in /usr/share/psa-horde/imp/login.php:
$title = sprintf(_("Welcome to %s"), $registry->get('name', ($imp_auth) ? 'horde' : null));
with
$rest = substr($_SERVER['SERVER_NAME'], 8);
$title = sprintf(_("Welcome to %s"),$rest);
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Both will make the greeting- "Welcome to domain.com" - with domain.com being whatever domain's webmail is being called.
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To remove (or replace) the default horde logo, edit the common-footer at /usr/share/psa-horde/templates/