Problem with Webmail search

Hi, We have roundcube client that when trying to search emails from a friend "Marko" using the search options (in inbox folder) he gets different results: If he search: "Mar" he gets this results: - Removed - When he search full name: "Marko" the results are fine: - Removed - How can we fix this in WHM or in Roundcube settings maybe ? We use dovecot (imap), maybe in those (custom) settings? Thanks, Viktor