authentication with IMAP server

I wrote a new auth module for authentication with remote IMAP servers. My hook_auth implementation returns TRUE but users can't login. Should I implement another hooks?

Any hint? This is the code:

 
...
 

/**
 * Implementation of hook_auth().
 */
function imapauth_auth($username, $password, $server) {
  
  //IMAP extension not loaded
  if (!function_exists('imap_open')) {
  	watchdog('php', t('IMAP extension not loaded. IMAP module couldn\'t be used to authenticate users.'), WATCHDOG_WARNING);
    return false;
  }

  if (variable_get('imapauth_enabled', 0) == 0) {
    return false;
  }
  
  $domains = split("/\r\n|\n|\r/", variable_get('imapauth_domains', array()));
 
  $valid = false;
  foreach ($domains as $d) {
    $domain_settings = split(",", $d, 2);
    
    if ((trim($domain_settings[0]) == $server) or trim($domain_settings[0]) == '*') {
      $valid = true;
      
      if (array_key_exists(1, $domain_settings)) {
        $mailbox = trim($domain_settings[1]);
      }
      else {
        $mailbox = "{" . $server . ":143}INBOX";
      }
    }
  }
  
  //This domain is not valid for IMAP authentication
  if (!$valid) {
      return false;
  }

  $mbox = @imap_open($mailbox, $username, $password);
  
  if ($mbox) {
		$minfo = @imap_mailboxmsginfo($mbox);
		
    if ($minfo) {
    	$login = true;
    }
    else {
    	$login = false;
    }
    @imap_close($mbox);
    

  } else {
  	$mbox = @imap_open($mailbox, $username . '@' . $server, $password);

    if ($mbox) {
  		$minfo = @imap_mailboxmsginfo($mbox);
		
      if ($minfo) {
      	$login = true;
      }
      else {
      	$login = false;
      }
      @imap_close($mbox);
    } 
    else {
    	$login = false;
    }
  }
  
  return $login;
}

...

/**
 * Implementation of hook_info().
 */

...

/**
 * Implementation of hook_settings().
 */

...

/**
 * Implementation of hook_help().
 */

...