I'm sure that you've noticed the Search box in the Kerio Connect Webmail Client:
What you may not have noticed is that it's more than just a place to type what you are looking for. If instead of starting to type, you click in the search box and press the Down-Arrow on your keyboard, you will get more search choices.
You can add to your selection by pressing Down-Arrow again:
Then, you can narrow your search more with your text:
If you you don't see the final selector, just press Down-Arrow again:
Don't forget that full text search is only available if it has been enabled in Administration!
If it has not been enabled (perhaps due to lack of disk space on the server), your searches do not look inside the text of messages and will only search in the header fields (addresses, subject).