Dovecot behind Load Balancer

Hi, i've a question for you. I've two dovecot imap/pop server behind a zen load balancer. Load balancing is made by lx4nat so the public ip address of my load balancer contact directly the dovecot servers. Since few months i've a message from thunderbird that i've reached the imap limit login for ip. I've triend to increase the max user ip parameter but sometimes i've the same problem. It's a strange things that actually i'm able to mitigate by reduce the number of cached connections in Thunderbird but it's not normal. What do you think about that? Maybe there's some tuning that you can suggest. Thank you, Paolo