diff --git a/snappymail/v/0.0.0/app/libraries/RainLoop/Actions/UserAuth.php b/snappymail/v/0.0.0/app/libraries/RainLoop/Actions/UserAuth.php
index 7030da219..82e811ac3 100644
--- a/snappymail/v/0.0.0/app/libraries/RainLoop/Actions/UserAuth.php
+++ b/snappymail/v/0.0.0/app/libraries/RainLoop/Actions/UserAuth.php
@@ -86,6 +86,11 @@ trait UserAuth
 			if (!\str_contains($sEmail, '@')) {
 				$sDefDomain = \trim($this->Config()->Get('login', 'default_domain', ''));
 				if (\strlen($sDefDomain)) {
+					if ('HTTP_HOST' === $sDefDomain || 'SERVER_NAME' === $sDefDomain) {
+						$sDefDomain = \preg_replace('/:[0-9]+$/D', '', $_SERVER[$sDefDomain]);
+					} else if ('gethostname' === $sDefDomain) {
+						$sDefDomain = \gethostname();
+					}
 					$this->Logger()->Write('Default domain "' . $sDefDomain . '" was used. (' . $sEmail . ' > ' . $sEmail . '@' . $sDefDomain . ')',
 						\LOG_INFO, 'LOGIN');
 
diff --git a/snappymail/v/0.0.0/app/libraries/RainLoop/Config/Application.php b/snappymail/v/0.0.0/app/libraries/RainLoop/Config/Application.php
index 17e524fa0..b0f1fe3a8 100644
--- a/snappymail/v/0.0.0/app/libraries/RainLoop/Config/Application.php
+++ b/snappymail/v/0.0.0/app/libraries/RainLoop/Config/Application.php
@@ -225,7 +225,12 @@ class Application extends \RainLoop\Config\AbstractConfig
 
 			'login' => array(
 
-				'default_domain' => array(''),
+				'default_domain' => array('',
+					'If someone logs in without "@domain.tld", this value will be used
+When this value is HTTP_HOST, the $_SERVER["HTTP_HOST"] value is used.
+When this value is SERVER_NAME, the $_SERVER["SERVER_NAME"] value is used.
+When this value is gethostname, the gethostname() value is used.
+'),
 
 				'allow_languages_on_login' => array(true,
 					'Allow language selection on webmail login screen'),