diff --git a/phpBB/includes/acp/acp_groups.php b/phpBB/includes/acp/acp_groups.php
index 60512c67b8..486616c33d 100644
--- a/phpBB/includes/acp/acp_groups.php
+++ b/phpBB/includes/acp/acp_groups.php
@@ -394,6 +394,15 @@ class acp_groups
 						}
 					}
 
+					// Validate the length of "Maximum number of allowed recipients per private message" setting.
+					// We use 16777215 as a maximum because it matches MySQL unsigned mediumint maximum value
+					// which is the lowest amongst DBMSes supported by phpBB3
+					if ($max_recipients_error = validate_data($submit_ary, array('max_recipients' => array('num', false, 0, 16777215))))
+					{
+						// Replace "error" string with its real, localised form
+						$error = array_merge($error, array_map(array(&$user, 'lang'), $max_recipients_error));
+					}
+
 					if (!sizeof($error))
 					{
 						// Only set the rank, colour, etc. if it's changed or if we're adding a new
diff --git a/phpBB/language/en/common.php b/phpBB/language/en/common.php
index bc38c1563d..e79e5a6de3 100644
--- a/phpBB/language/en/common.php
+++ b/phpBB/language/en/common.php
@@ -566,6 +566,9 @@ $lang = array_merge($lang, array(
 	'TEST_CONNECTION'	=> 'Test connection',
 	'THE_TEAM'			=> 'The team',
 	'TIME'				=> 'Time',
+	
+	'TOO_LARGE'						=> 'The value you entered is too large.',
+	'TOO_LARGE_MAX_RECIPIENTS'		=> 'The value of <strong>Maximum number of allowed recipients per private message</strong> setting you entered is too large.',
 
 	'TOO_LONG'						=> 'The value you entered is too long.',
 
@@ -608,6 +611,9 @@ $lang = array_merge($lang, array(
 	'TOO_SHORT_EMAIL_CONFIRM'		=> 'The e-mail address confirmation you entered is too short.',
 	'TOO_SHORT_WEBSITE'				=> 'The website address you entered is too short.',
 	'TOO_SHORT_YIM'					=> 'The Yahoo! Messenger name you entered is too short.',
+	
+	'TOO_SMALL'						=> 'The value you entered is too small.',
+	'TOO_SMALL_MAX_RECIPIENTS'		=> 'The value of <strong>Maximum number of allowed recipients per private message</strong> setting you entered is too small.',
 
 	'TOPIC'				=> 'Topic',
 	'TOPICS'			=> 'Topics',